UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 14A

INFORMATION REQUIRED IN PROXY STATEMENT

SCHEDULE 14A INFORMATION

Proxy Statement Pursuant to Section 14(a) of the

Securities Exchange Act of 1934

Filed by the Registrant ☒

Filed by a Party other than the Registrant ☐

Check the appropriate box:

William Blair Funds

(Name of Registrant as Specified in Its Charter)

Payment of Filing Fee (Check the appropriate box):

WILLIAM BLAIR FUNDS

WILLIAM BLAIR LARGE CAP GROWTHSHORT DURATION BOND FUND

(FORMERLY, WILLIAM BLAIR INCOME FUND)

150 NORTH RIVERSIDE PLAZA

CHICAGO, ILLINOIS 60606

September 30, 2020December 13, 2021

Dear Shareholder,

You are cordially invited to attend a special meeting of the shareholders of William Blair Large Cap GrowthShort Duration Bond Fund (formerly known as William Blair Income Fund and William Blair Short-Term Bond) (the “Fund”), a series of William Blair Funds (the “Trust”),. At its October 26-27, 2021 meeting, the Board approved a change in the Fund’s name from the “William Blair Income Fund” to the “William Blair Short-Term Bond Fund”. Subsequently in December 2021, the Board approved a change in the Fund’s name from the “William Blair Short-Term Bond Fund” to the “William Blair Short Duration Bond Fund”. The special meeting of the shareholders of the Fund will be held virtually on November 12, 2020January 27, 2022 at 12:00 p.m. Central time (the “Meeting”). The Meeting will be conducted as a virtual meeting hosted by means of a live webcast. The Board of Trustees of the Trust has implemented a virtual meeting format primarily to reflect our and global concerns regarding the spread of COVID-19. Shareholders will be able to listen, vote, and submit questions from their home or any location with internetInternet connectivity.

At the Meeting, you will be asked to consider and act upon the following proposals:proposals (the “Proposals”):

The following Q&A is provided to assist you in understanding the proposals.Proposals. The Proposal isProposals are described in greater detail in the enclosed proxy statement.

Your vote is important. Whether or not you expect to attend the Meeting, it is important that your shares be represented. Your immediate response will help reduce the need for the Fund to conduct additional proxy solicitations. Please review the proxy statement and then vote by Internet, telephone or mail as soon as possible. If you vote by mail, please sign and return all of the proxy cards included in this package.

Sincerely,

IMPORTANT INFORMATION TO HELP YOU UNDERSTAND

AND VOTE ON THE PROPOSALPROPOSALS

While we strongly encourage you to read the full text of the enclosed Proxy Statement, we are also providing you with a brief overview of the subject of the shareholder vote. Your vote is important.

QUESTIONS AND ANSWERS

Q. What proposal am I being asked to vote on?

A. At a special meeting of shareholders (the “Meeting”) of William Blair Large Cap GrowthShort Duration Bond Fund (formerly known as William Blair Income Fund and William Blair Short-Term Bond Fund) (the “Fund”), you will be asked to vote on the following proposalproposals with respect to the Fund, and to transact any other business as may properly come before the Meeting or any adjournment or postponement thereof: Toto approve a change in the Fund’s classification underfundamental investment objective to indicate that the Investment Company Act of 1940 (the “1940 Act”) from a “diversified” fundFund seeks to a “non-diversified” fund (the “Proposal”).

Q. What is happening?

A. Section 5(b)(1) of the 1940 Act requires every mutual fund to elect to be classified as either a “diversified” fund or “non-diversified” fund within the meaning of the 1940 Act. A fund that elects to be a diversified fund under Section 5(b)(1) of the 1940 Act must, with respect to 75 percent of its total assets, limit its investment in the securities of any one issuer (with certain limited exceptions). By contrast, a non-diversified fund may invest in greater proportions in the securities of fewer issuersgenerate current income, rather than a diversified fund. The 1940 Act also requires shareholdershigh level of current income and to approve a change in fundamental investment policy for the Fund to remove, as a fund’s classification fromfundamental policy, the current restriction on investment in fixed income securities rated below a diversified fund to a non-diversified fund.specified rating level by nationally recognized rating organizations as described in the proxy statement (the “Proposals”).

Q. Why are the Proposals being recommended?

A. The marketplace for short-term, investment grade, fixed income funds has evolved since the inception of the Fund. William Blair Investment Management, LLC (the “Adviser” or “WBIM”), the Fund’s investment adviser, believes that under current market conditions classification ascertain changes to the Fund’s investment objective and fundamental investment policies could enable the Adviser to manage the Fund more effectively and in a non-diversified fund would be beneficial in executingmanner more consistent with investor needs and reflective of the Adviser’s strategy forgrowth and structure of the Fund.bond market. Therefore, shareholders are being asked to approve a changethese changes in the Fund’s classification from a diversified fundinvestment objective and fundamental investment policy. Under the Investment Company Act of 1940 (the “1940 Act”), changes to a fund’s fundamental investment policy or to a fund’s fundamental investment objective require shareholder approval.

Q. What revisions are being made to the fundamental investment objective of the Fund?

A.Currently the objective of the Fund is:

The William Blair Short Duration Bond Fund seeks a high level of current income with relative stability of principal.

The Adviser recommends changing the objective to read as follows:

The William Blair Short Duration Bond Fund seeks to generate current income, consistent with relative stability of capital.

The Adviser believes this change would align the Fund’s investment objective with the expected future management of the Fund and management’s understanding of shareholder investment expectations for short-term bond funds. If Proposal 1 is approved by shareholders, the Fund’s investment objective would continue to be fundamental, meaning it could not be changed further in the future without shareholder approval.

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Q. What revisions are being made to the fundamental investment policy of the Fund?

A.The Fund may currently invest up to 10% of its total assets in below non-diversified“A-” fund,rated debt. To more closely align with the portfolio management team’s expectations for managing the Fund and in light of growth in the percentage of investment grade debt represented by BBB-rated debt (including debt rated “BBB+”, “BBB”, and “BBB-”), the investment team believes it is appropriate to have additional flexibility to be able to invest a larger portion of the Fund’s total assets in a wider range of debt, including debt rated “BBB+” or lower, to help balance the Fund’s exposure between credit spreads and interest rates. The Adviser expects that any increase in BBB-rated debt will not materially diverge from the Fund’s current risk/return profile.

The Fund currently has the following fundamental investment policy:

“As a matter of fundamental policy, under normal market conditions, the Fund invests at least 90% of its total assets in the following: (a) U.S. dollar-denominated corporate debt securities (domestic or foreign) with long-term ratings of “A-” or better, or an equivalent rating, by at least one of the following three nationally recognized statistical rating organizations: Fitch Ratings, Moody’s Investors Service, Inc. and Standard & Poor’s, a division of The McGraw-Hill Companies, Inc. (“Rating Organizations”); (b) obligations of or guaranteed by the U.S. Government, its agencies or instrumentalities; (c) collateralized obligations, which are debt securities issued by a corporation, trust or custodian, or by a U.S. Government agency or instrumentality, that are collateralized (i.e., secured as definedto payment of interest and/or principal) by a portfolio or pool of assets, such as mortgages, mortgage-backed securities, debit balances on credit card accounts or U.S. Government securities (the Fund may invest in collateralized obligations that are not guaranteed by a U.S. Government agency or instrumentality only if the collateralized obligations are rated “A-” or better, or an equivalent rating, by one of the Rating Organizations); and (d) commercial paper obligations rated within the highest grade by one of the Rating Organizations.”

It is proposed that the fundamental investment policy by revised to read as follows:

As a matter of fundamental policy, under normal market conditions, the Fund invests at least 80% of its net assets (plus the amount of any borrowings for investment purposes) in bonds.

The revised fundamental investment policy will provide the Fund with greater flexibility to invest in debt of any quality, including BBB-rated debt securities (including debt securities rated “BBB+”, “BBB”, and “BBB-”), which, as described in the accompanying proxy statement, will help the Fund balance its exposure between credit spreads and interest rates and will also satisfy the regulatory requirement that the Fund adopt a policy to invest at least 80% of its net assets in bonds consistent with its new name. For purposes of the policy, “bonds” would include all types of fixed income instruments.

Assuming that the fundamental investment policy is revised as indicated above, the Adviser would also include the following additional disclosure in the Fund’s principal investment strategies:

“The Fund invests primarily in U.S. dollar denominated, investment grade fixed income securities. A security is considered to be investment grade if it is rated in one of the highest four categories by at least one nationally recognized statistical rating organization (“Rating Organization”) at the time of investment.”

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Proposal 2 could incrementally increase the risk profile of the Fund. The lower the rating of a bond, the greater the risk is that the issuer could default on its obligation to make payments under the 1940 Act.bond. The Adviser notes however that any security rated “BBB-” or higher is still considered investment grade.

The Adviser does not intend to purchase securities for the Fund that are rated below “BBB-” at the time of purchase. However, securities that are downgraded below “BBB-” after purchase may continue to be held by the Fund. This policy is not a fundamental investment policy and may be changed in the future by the Board without shareholder approval.

Increasing the amount the Fund may invest in below “A-” rated debt allows for greater diversification within the Fund. Fixed income investors can earn returns primarily through two sources: credit spreads and interest rates. The Adviser intends to manage the weighted average duration (a measure of interest rate/sensitivity) of the Fund along with investing in a broader range of bonds carrying different credit ratings. Duration is a measure of interest rate risk, or the sensitivity of an investment to changes in interest rates. For example, if an investment has a duration of five years, for every 1% increase (or decrease) in interest rates, the price of a security is expected to fall (or rise) by approximately 5%. The anticipated weighted average duration for the Fund is up to 3.5 years. Although the portfolio repositioning associated with transitioning from the Fund’s current fundamental investment policy, if approved by shareholders, may result in tax or other costs to the Fund, the Adviser does not expect any such costs to be material.

Q. Why are you sending me this information?

A. You are receiving these proxy materials because you own shares of the Fund and have the right to vote on thisthese very important proposalproposals concerning your investment.

Q. What are the benefits and risks of the change of the Fund’s classification from a “diversified” fund to a “non-diversified” fund?

A. The Adviser believes that changing the Fund’s classification to non-diversified will benefit the Fund by providing the Fund the flexibility to hold larger positions in the securities of certain companies. The Adviser intends to use this increased investment flexibility to take larger positions in the securities of fewer issuers when it believes doing so will benefit the Fund in pursuing its investment objective. It is important to note that, to the extent the Fund invests its assets in a smaller number of issuers, the Fund will be more susceptible to negative events affecting those issuers than a diversified fund. Investing a larger percentage of the Fund’s assets in any one issuer could increase the Fund’s risk of loss and its share price volatility because the value of its shares would be more susceptible to adverse events affecting that issuer.

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Q. Has the Board of Trustees (the “Board”) of William Blair Funds approved the Fund’s change in classification from “diversified” to “non-diversified”Proposals and how does the Board recommend that I vote?

A. The Board unanimously approved the change in the Fund’s classification from a “diversified” fund to a “non-diversified” fundProposals at a meeting held on September 14, 2020,October 26-27, 2021 and recommends that you voteFOR the Proposal.Proposals.

Q. Who will bear the costs related to this proxy solicitation?

A. The cost of the proxy solicitation will be borne by the Fund,Fund; however, by virtue of the operation of the expense limitation arrangements currently in effect with respect to the Fund, these costs will ultimately be borne by WBIM. Broadridge Financial Services, Inc. (“Broadridge”) has been engaged to assist in the solicitation of proxies for the TrustFund at an estimated cost of $55,000,$40,000, plus reimbursement for reasonable expenses. However, the exact cost will depend on the amount and types of services rendered.

Q. Who is entitled to vote?

A. If you owned shares of the Fund as of the close of business on September 25, 2020November 19, 2021 (the “Record Date”), you are entitled to vote those shares.

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Q. When and where will the Meeting be held?

A. After considering the continuing health impacts of COVID-19, related governmental orders and guidance, and the wellbeing of shareholders, employees, and communities, the Board has determined to hold the Meeting only by means of remote communication through a live internet webcast. An in-person meeting at a physical location will not be held.

Q. How do I vote my shares?

A. For your convenience, there are several ways you can vote:

Q. What vote is required to approve the Proposal?Proposals?

A. Approval of theeach Proposal requires the affirmative vote of a “majority of the outstanding voting securities” of the Fund, which under the 1940 Act means an affirmative vote of the lesser of (a) 67% or more of the shares of the Fund present at the Meeting or represented by proxy if the holders of more than 50% of the outstanding shares are present or represented by proxy, or (b) more than 50% of the outstanding shares of the Fund. For purposes of the Proposals, all shareholders vote together, regardless of which class they own.

The Proposals are independent of each other. Therefore, it may be the case that one Proposal may be approved while another is not, in which case only the Proposal that is approved will be acted on.

Q. What happens if I sign and return my proxy card but do not mark my vote?

A. Your proxy will be voted in favor of the Proposal.Proposals.

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Q. May I revoke my proxy?

A. You may revoke your proxy at any time before it is exercised at the meeting by giving notice of your revocation to the Fund in writing, or by the execution and delivery of a later-dated proxy. You may also revoke your proxyYour notice of revocation must be received by attending and voting at the Meeting.January 25, 2022 no later than 10:59 p.m. Central time.

Q. Whom should I call for additional information about this Proxy Statement?

A. If you need any assistance or have any questions regarding the ProposalProposals or how to vote your shares, please call the Fund’s proxy information line at 1-833-934-2740 and follow the recorded instructions.

THE ATTACHED PROXY STATEMENT CONTAINS MORE DETAILED INFORMATION ABOUT THE PROPOSAL.PROPOSALS. PLEASE READ IT CAREFULLY. YOUR VOTE IS IMPORTANT.

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WILLIAM BLAIR FUNDS

WILLIAM BLAIR LARGE CAP GROWTHSHORT DURATION BOND FUND

(FORMERLY, WILLIAM BLAIR INCOME FUND)

150 NORTH RIVERSIDE PLAZA

CHICAGO, ILLINOIS 60606

NOTICE OF SPECIAL MEETING OF SHAREHOLDERS

TO BE HELD NOVEMBER 12, 2020JANUARY 27, 2022

September 30, 2020December 13, 2021

To the Shareholders:

You are invited to attend a special meeting of the shareholders of William Blair Large Cap GrowthShort Duration Bond Fund (formerly known as William Blair Income Fund and William Blair Short-Term Bond Fund) (the “Fund”), a series of William Blair Funds (the “Trust”),. At its October 26-27, 2021 meeting, the Board approved a change in the Fund’s name from the “William Blair Income Fund” to the “William Blair Short-Term Bond Fund”. Subsequently in December 2021, the Board approved a change in the Fund’s name from the “William Blair Short-Term Bond Fund” to the “William Blair Short Duration Bond Fund”. The special meeting of the shareholders of the Fund will be held virtually on November 12, 2020January 27, 2022 at 12:00 p.m. Central time (the “Meeting”) for the following purposes:

The Board of Trustees of the Trust has fixed the close of business on September 25, 2020November 19, 2021 as the record date for determining the shareholders of the Fund entitled to notice of and to vote at the meetingMeeting or any adjournments or postponements thereof. Shareholders are entitled to one vote for each share held and a proportionate fractional vote for each fractional share held.

You will be able to attend the Meeting online, submit your questions during the Meeting and vote your shares electronically. To participate in the Meeting, shareholders must register in advance by visiting www.williamblairfunds.comhttps://www.viewproxy.com/WilliamBlairFund/broadridgevsm/ and submitting the required information to the Adviser.William Blair Investment Management, LLC (the “Adviser” or “WBIM”).

Shareholders whose shares are registered directly with the Fund in the shareholder’s name will be asked to submit their name and control number found on the shareholder’s proxy card in order to register to participate in and vote at the Meeting. Shareholders whose shares are held by a broker, bank or other nominee must first obtain a “legal proxy” from the applicable nominee/record holder, who will then provide the shareholder with a newly-issued control number. We note that obtaining a legal proxy may take several days. Requests for registration should be received no later than 4:00 p.m., Central time, on November 10, 2020,January 12, 2022, but in any event must be received by the scheduled time for commencement of the Meeting. Once shareholders have obtained a new control number, they must visit www.williamblairfunds.comhttps://www.viewproxy.com/WilliamBlairFund/broadridgevsm/ and submit their name and newly-issued control number in order to register to participate in and vote at the Meeting.

After shareholders have submitted their registration information, they will receive an email that confirms that their registration request has been received and is under review by the Adviser. Once a shareholder’s registration request has been accepted, the shareholder will receive (i) an email containing an event link and dial-in information to attend the Meeting, and (ii) an email with a password to enter at the event link in order to access the Meeting. Shareholders may vote before or during the Meeting at www.williamblairfunds.com.https://www.viewproxy.com/WilliamBlairFund/broadridgevsm/. Only shareholders of the Fund present virtually or by proxy will be able to vote, or otherwise exercise the powers of a shareholder, at the Meeting.

The Meeting webcast will begin promptly at 12:00 p.m. Central time. We encourage you to access the Meeting prior to the start time. For additional information on how you can attend and

participate in the virtual Meeting, please see the instructions beginning on the first page of the proxy statement that follows. Because the Meeting will be a completely virtual meeting, there will be no physical location for shareholders to attend.

By order of the Board of Trustees

In order to avoid delay and additional expense and to assure that your shares are represented, please vote as promptly as possible, regardless of whether or not you plan to attend the meeting. You may vote by mail, telephone or over the Internet. To vote by mail, please mark, sign, date and mail the enclosed proxy card. No postage is required if mailed in the United States. To vote by telephone, please call the toll-free number located on your proxy card and follow the recorded instructions, using your proxy card as a guide. To vote over the Internet, go to the Internet address provided on your proxy card and follow the instructions, using your proxy card as a guide.

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PROXY STATEMENT

WILLIAM BLAIR FUNDS

WILLIAM BLAIR LARGE CAP GROWTHSHORT DURATION BOND FUND

(FORMERLY, WILLIAM BLAIR INCOME FUND)

150 NORTH RIVERSIDE PLAZA

CHICAGO, ILLINOIS 60606

September 30, 2020December 13, 2021

Important Notice Regarding the Availability of Proxy Materials for the Shareholder Meeting to be Held on November 12, 2020:January 27, 2022:

The Proxy Materials are available at: www.ProxyVote.com

GENERAL

The accompanying proxy is solicited by the Board of Trustees of William Blair Funds (the “Board” and each member thereof a “Trustee”) for voting at the special meeting of shareholders of William Blair Large Cap GrowthShort Duration Bond Fund (formerly known as William Blair Income Fund and William Blair Short-Term Bond Fund) (the “Fund”), a series of William Blair Funds (the “Trust”),. At its October 26-27, 2021 meeting, the Board approved a change in the Fund’s name from the “William Blair Income Fund” to the “William Blair Short-Term Bond Fund”. Subsequently in December 2021, the Board approved a change in the Fund’s name from the “William Blair Short-Term Bond Fund” to the “William Blair Short Duration Bond Fund”. The special meeting of the shareholders of the Fund will be held virtually on November 12, 2020January 27, 2022 at 12:00 p.m. Central time and at any and all adjournments or postponements thereof (the “Meeting”). This proxy statement and the enclosed proxy are first being mailed to shareholders on or about October 9, 2020.

At the Meeting, shareholders are being asked to consider and act upon the following proposals:December 16, 2021.

The Meeting will be a virtual meeting conducted exclusively via live webcast starting at 12:00 p.m. Central time. You will be able to attend the Meeting online, submit your questions during the Meeting and vote your shares. To participate in the Meeting, shareholders must register in advance by visiting www.williamblairfunds.comhttps://www.viewproxy.com/WilliamBlairFund/broadridgevsm/ and submitting the required information to the Adviser.William Blair Investment Management, LLC (the “Adviser” or “WBIM”).

Shareholders whose shares are registered directly with the Fund in the shareholder’s name will be asked to submit their name and control number found on the shareholder’s proxy card in order to register to participate in and vote at the Meeting. Shareholders whose shares are held by a broker, bank

or other nominee must first obtain a “legal proxy” from the applicable nominee/record holder, who will then provide the shareholder with a newly-issued control number. We note that obtaining a legal proxy may take several days. Requests for registration should be received no later than 4:00 p.m., Central time, on November 10th, 2020,January 12, 2022, but in any event must be received by the scheduled time for commencement of the Meeting. Once shareholders have obtained a new control number, they must visit www.williamblairfunds.comhttps://www.viewproxy.com/WilliamBlairFund/broadridgevsm/ and submit their name and newly-issued control number in order to register to participate in and vote at the Meeting.

After shareholders have submitted their registration information, they will receive an email that confirms that their registration request has been received and is under review by the Adviser. Once a shareholder’s registration request has been accepted, the shareholder will receive (i) an email containing an event link and dial-in information to attend the Meeting, and (ii) an email with a password to enter at the event link in order to access the Meeting. Shareholders may vote before or during the Meeting at www.williamblairfunds.com.https://www.viewproxy.com/WilliamBlairFund/broadridgevsm/. Only shareholders of the Fund present virtually or by proxy will be able to vote, or otherwise exercise the powers of a shareholder, at the Meeting.

In light of the rapidly changing developments related to COVID-19, we are pleased to offer our shareholders a completely virtual Meeting, which provides worldwide access and communication, while protecting the health and safety of our shareholders and the Adviser’s personnel.Fund’s officers. We are committed to ensuring that shareholders will be afforded the same rights and opportunities to participate as they would at an in-person meeting. We will try to answer as many shareholder-submitted questions as time permits that comply with the Meeting rules of conduct. However, we reserve the right to edit profanity or other inappropriate language, or to exclude questions that are not pertinent to meeting matters or that are otherwise inappropriate. If substantially similar questions are received, we will group such questions together and provide a single response to avoid repetition.

Shareholders Entitled to Vote

The Board has fixed the close of business on September 25, 2020November 19, 2021 as the record date (the “Record Date”) for the determination of shareholders entitled to notice of and to vote at the Meeting. As of the Record Date, 31,923,3764,715,655 shares of the Fund were issued and outstanding in the following series and classes:

Required Vote

A quorum of shareholders is required to take action at the Meeting. Holders of one-third of the outstanding shares of the Fund, present in person or by proxy, will constitute a quorum of shareholders at the Meeting. If a quorum is present at the Meeting, then the affirmative vote of a majority“majority of the outstanding voting securitiessecurities” of the Fund, as defined in the Investment Company Act of 1940 (the “1940 Act”) is required for shareholders to approve the Proposal.Proposals. A “majority of the outstanding voting securities” is defined in the 1940 Act as the lesser of (a) 67% or more of the shares present at the Meeting, if the holders of more than 50% of the outstanding shares of the Fund are present at the Meeting or represented by proxy, or (b) more than 50% of the outstanding shares of the Fund.

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If a proxy card is properly executed and returned accompanied by instructions to withhold authority (an abstention), the shares represented thereby will be counted as shares present and entitled to vote for purposes of determining whether a quorum is present, but will not be counted as a vote in favor of the Proposal.Proposals. Accordingly, abstentions effectively will be a vote against the Proposal.Proposals. Ordinarily, broker non-votes, if any, would be counted as shares present and entitled to vote for purposes of determining whether a quorum is present, but would not be counted as a vote in favor of the Proposal.Proposals. However, because the Proposal isProposals are considered non-routine, broker non-votes are inapplicable to this solicitation and will have no impact on establishing quorum or the votes cast for or against the Proposal.Proposals.

Each valid proxy will be voted in accordance with the instructions provided on the proxy and as the persons named in the proxy determine on such other business as may come before the Meeting. If no instructions are given, the proxy will be voted FOR the Proposal.Proposals. Shareholders are entitled to one vote for each share held and a proportionate fractional vote for each fractional share held. Shareholders may vote by filling out and signing the enclosed proxy card and returning it in the postage paid envelope provided. Shareholders may also vote by telephone or over the Internet. Please see the instructions on your proxy card for telephone voting and Internet voting. Shareholders will have an opportunity to review their voting instructions and make any necessary changes before submitting their

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voting instructions and terminating their telephone call or Internet link. Shareholders who execute a proxy in any of the above manners may revoke their proxy at any time before their shares are voted, either by sending the Trust a written notice of revocation or by delivering a duly executed proxy bearing a later date, ordate. Your notice of revocation must be received by attending and voting at the Meeting.January 25, 2022 no later than 10:59 p.m. Central time.

PROPOSALPROPOSALS

TO APPROVE A CHANGE IN THE FUND’S CLASSIFICATION UNDER THE 1940 ACT FROM A “DIVERSIFIED” FUND TO A “NON-DIVERSIFIED” FUNDFUNDAMENTAL INVESTMENT OBJECTIVE AND FUNDAMENTAL INVESTMENT POLICY

Background and Description of the ProposalProposals

The purposemarketplace for short-term, investment grade, fixed income funds has evolved since the inception of the Proposal isFund. The Adviser believes that certain changes to change the Fund’s classification soinvestment objective and fundamental investment policies could enable the Adviser to manage the Fund more effectively and in a manner more consistent with investor needs and reflective of the growth and structure of the bond market. Therefore, shareholders are being asked to approve these changes to the Fund’s fundamental investment objective and fundamental investment policy. Under the Investment Company Act of 1940 (the “1940 Act”), changes to a fund’s fundamental investment policy or to a fund’s fundamental investment objective require shareholder approval.

Proposal 1 – To approve a change to the Fund’s Fundamental Investment Objective

Proposal 1 relates to a proposed change to the Fund’s fundamental investment objective. Currently the objective of the Fund is:

The William Blair Short Duration Bond Fund seeks a high level of current income with relative stability of principal.

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The Adviser recommends changing the objective to read as follows:

The William Blair Short Duration Bond Fund seeks to generate current income, consistent with relative stability of capital.

The Adviser believes this change would align the Fund’s investment objective with the expected future management of the Fund and management’s understanding of shareholder investment expectations for short-term bond funds. If shareholders approve Proposal 1, the Adviser will operate the Fund under the revised investment objective. The Fund’s investment objective would continue to be fundamental, meaning it could not be changed further in the future without shareholder approval. If shareholders do not approve Proposal 1, the Fund will continue to be managed with the investment objective of seeking a high level of current income with relative stability of principal.

Proposal 2 – To approve changes to certain of the Fund’s Fundamental Investment Policy

Proposal 2 relates to a proposed change to the Fund’s fundamental investment policy with respect to the credit ratings of the debt securities in which the Fund currently invests at least 90% of its assets. The Adviser believes that if approved, Proposal 2 would provide the Adviser more flexibility to invest the Fund’s assets in a manner that could result in improved investment performance for the Fund.

The Fund may currently invest up to 10% of its total assets in below “A-” rated debt. To more closely align with the portfolio management team’s expectations for managing the Fund and in light of growth in the percentage of investment grade debt represented by BBB-rated debt (including debt rated “BBB+”, “BBB”, and “BBB-”), the investment team believes it is appropriate to have additional flexibility to be able to invest a larger portion of the Fund’s total assets in a wider range of debt, including debt rated “BBB+” or lower, to help balance the Fund’s exposure between credit spreads and interest rates. The Adviser expects that any increase in BBB-rated debt will not materially diverge from the Fund’s current risk/return profile. As set forth in more detail below, if Proposal 2 is approved by shareholders, the Fund would be allowedsubject to operate as a “non-diversified” fund underan investment policy to invest primarily in U.S. dollar denominated, investment grade fixed income securities and the 1940 Act. If shareholders approve this proposal,Adviser would not intend to purchase securities for the Fund wouldthat are rated below “BBB-” at the time of purchase. However, these policies are not fundamental investment policies of the Fund and, as a result, may be able to focus its investments more heavilychanged in securities of fewer issuers in which the Fund’s portfolio managers have a higher conviction.future by the Board without shareholder approval.

The Fund currently is classified as a “diversified” fund for purposes of Section 5(b)(1) underhas the 1940 Act. following principal investment strategies:

“As a diversified fund, with respect to 75%matter of fundamental policy, under normal market conditions, the Fund invests at least 90% of its total assets in the following: (a) U.S. dollar-denominated corporate debt securities (domestic or foreign) with long-term ratings of “A-” or better, or an equivalent rating, by at least one of the following three nationally recognized statistical rating organizations: Fitch Ratings, Moody’s Investors Service, Inc. and Standard & Poor’s, a division of The McGraw-Hill Companies, Inc. (“Rating Organizations”); (b) obligations of or guaranteed by the U.S. Government, its agencies or instrumentalities; (c) collateralized obligations, which are debt securities issued by a corporation, trust or custodian, or by a U.S. Government agency or instrumentality, that are collateralized (i.e., secured as to payment of interest and/or principal) by a portfolio or pool of assets, such as mortgages, mortgage-backed securities, debit balances on credit card accounts or U.S. Government securities (the Fund may invest in collateralized obligations that are not guaranteed by a U.S. Government agency or instrumentality only if the

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collateralized obligations are rated “A-” or better, or an equivalent rating, by one of the Rating Organizations); and (d) commercial paper obligations rated within the highest grade by one of the Rating Organizations.”

It is proposed that the fundamental investment policy be revised to read as follows:

As a matter of fundamental policy, under normal market conditions, the Fund invests at least 80% of its net assets (plus the amount of any borrowings for investment purposes) in bonds.

The revised policy will provide the Fund with greater flexibility to invest in debt of any quality, including BBB-rated debt securities (including debt securities rated “BBB+”, “BBB”, and “BBB-”), which as described below, will help the Fund balance its exposure between credit spreads and interest rates and will also satisfy the regulatory requirement that the Fund adopt a policy to invest at least 80% of its net assets in bonds consistent with its new name. For purposes of the policy, “bonds” would include all types of fixed income instruments.

Assuming that Proposal 2 is approved by shareholders, the Adviser would also include the following additional disclosure in the Fund’s principal investment strategies(although this is not a fundamental policy and could be changed in the future without shareholder approval):

“The Fund invests primarily in U.S. dollar denominated, investment grade fixed income securities. A security is considered to be investment grade if it is rated in one of the highest four categories by at least one nationally recognized statistical rating organization (“Rating Organization”) at the time of investment.”

The Fund also currently has a non-fundamental investment policy that up to 10% of the Fund’s total assets may be invested in debt securities that at the time of purchase are rated lower than “A-” but at least “BBB-” (or its equivalent) by at least one nationally recognized statistical rating organization, so long as the Fund does not invest in a security if, as a result of such investment, more than 5%3% of its total net assets in securities of any single issuer whose securities are rated “BBB-”. Securities that are downgraded below “BBB-” (or its equivalent) after purchase may continue to be held in the Fund.

If Proposal 2 is approved by shareholders, this non-fundamental investment policy would be updated to (i) permit the Fund to invest up to 100% of its total assets (calculatedin in debt securities that at the time of purchase) would be investedpurchase are rated at least “BBB-” (or its equivalent) by at least one nationally recognized statistical rating organization and (ii) eliminate the restriction on investing more than 3% of the Fund’s total net assets in securities of any one issuer. In addition, with respectsingle issuer whose securities are rated “BBB-”.

The Adviser does not intend to 75% of its total assets,purchase securities for the Fund that are rated below “BBB-” at the time of purchase. However, securities that are downgraded below “BBB-” after purchase may continue to be held by the Fund. This policy is not hold more than 10%a fundamental policy and may be changed in the future by the Board without shareholder approval.

Proposal 2 could incrementally increase the risk profile of the outstanding voting securitiesFund. The lower the rating of a bond, the greater the risk is that the issuer could default on its obligation to make payments under the bond. The Adviser notes however that any one issuer. The restrictions in Section 5(b)(1) do not apply to U.S. government securities, securities of othersecurity rated “BBB-” or higher is still considered investment companies, cash and cash items (including receivables). As to the remaining 25% of total assets, there is no limitation ongrade.

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Increasing the amount of assets the Fund may invest in a single issuer.below “A-” rated debt allows for greater diversification within the Fund. Fixed income investors can earn returns primarily through two sources: credit spreads and interest rates. The Fund’s current classification as a “diversified” fund underAdviser intends to manage the 1940 Act is a fundamental policyweighted average duration (a measure of interest rate risk/sensitivity) of the Fund, which means that it cannot be changed without shareholder approval.along with investing in a broader range of bonds carrying different credit ratings. Duration is a measure of interest rate risk, or the sensitivity of an investment to changes in interest rates. For example, if an investment has a duration of five years, for every 1% increase (or decrease) in interest rates, the price of a security is expected to fall (or rise) by approximately 5%. The Board of Trustees, including all ofanticipated weighted average duration for the Trustees who are not “interested persons” ofFund is up to 3.5 years. Although the Trust as defined in the 1940 Act (the “Independent Trustees”), recommends that shareholders vote to changeportfolio repositioning associated with transitioning from the Fund’s classificationcurrent fundamental investment policy, if approved by shareholders, may result in tax or other costs tonon-diversified.

Generally, diversification reduces risk and non-diversification increases risk. You are being asked to change the Fund, from diversified to non-diversified, or to make it potentially a riskier fund.

This proposal is intended to empower William Blair Investment Management, LLC, the Fund’s investment adviser (the “Adviser” or “WBIM”), to decide to allocate risk in relation to expected return with the freedom to invest larger portions of the Fund’s portfolio in a single issuer. Though it is an unlikely scenario, in theory the Adviser could potentially invest updoes not expect any such costs to 25% of the Fund’s assets in two issuers if the Proposal is approved.be material.

The main purpose of this flexibility is toproposed changes will allow the Fund to take over 5% positionsAdviser greater flexibility in single issuers that in the aggregate exceed 25% of Fund assets. The Adviser believes that this increased investment flexibility may provide opportunities to enhance the Fund’s performance. Of course, if the Adviser invests a significant percentage of the Fund’s assets in a single issuer, the Fund’s performance would be closely tied to the market value of that issuer, and could be more volatile (riskier) than the performance of diversified funds. Investing a larger percentage of the Fund’s assets in a single issuer’s securities increasesbalancing the Fund’s exposure tobetween credit spreads and other risks associated with that issuer’s financial condition and business operations. The Adviser will use this increased flexibility to acquire and hold

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larger positions in the securities of a single issuer only if and when it believes doing so justifies the risks involved. The Adviser believes this flexibility would allow the Adviser to align the Fund’s portfolio better with the investment team’s philosophy and portfolio construction process, and will reduce differences with other vehicles and separate accounts managed according to the same investment strategy.interest rates.

If shareholders approve this proposal,Proposal 2, the Adviser maywill operate the Fund as non-diversified or it may not. The Adviser will reserveunder the freedom of action to operate the Fund as non-diversified only if and when the Adviser believes it would be in shareholders’ best interests to do so, provided that if the Adviser does not operate the Fund as non-diversified within three years of shareholder approval, 1940 Act rules will require the Adviser to again seek shareholder approval to reserve the freedom of action to operate the Fund as non-diversified.

It should be noted that, although the Fund would no longer be subject to the 1940 Act diversification restrictions if shareholders approve the Proposal at the Meeting, the Fund intends to continue to comply with Federal tax diversification restrictions of Subchapter M of the Internal Revenue Code of 1986 (the “IRC”). For purposes of the IRC, the Fund operates as a “regulatedrevised fundamental investment company.” Under the IRC, the Fund must diversify its holdings so that, in general, at the close of each quarter of its taxable year, (i) at least 50% of the fair market value of the Fund’s total (gross) assets is comprised of cash, cash items, U.S. government securities, securities of other regulated investment companies and other securities limited in respect of any one issuer to an amount not greater in value than 5% of the value of the Fund’s total assets and to not more than 10% of the outstanding voting securities of such issuer; and (ii) not more than 25% of the value of its total (gross) assets is invested in the securities of any one issuer (other than U.S. government securities and securities of other regulated investment companies), two or more issuers controlled by the Fund and engaged in the same, similar or related trades or businesses, or certain publicly traded partnerships. These Federal tax diversification requirements, or the Fund’s determination to comply with them, may change in the future without shareholder approval.

The proposal would, as a legal matter, also allow the Fund to hold a greater number of over 10% positions in the voting securities of an issuer. The Adviser does not currently expect that approval of this proposal will materially affect the way in which the Fund is managed with regard to the number of over 10% positions.

policy. If shareholders of the Fund do not approve the Proposal 2, the Fund will continue to operate under its existing fundamental investment policy as a “diversified” fund.described in the Fund’s current registration statement and as summarized in this proxy statement.

Review of the ProposalProposals and Recommendation of the Board

At a meeting held on September 14, 2020,October 26-27, 2021, the Board considered the ProposalProposals and voted unanimously to approve the Proposal.Proposals. In determining to recommend approval ofthat shareholders vote to approve the Proposal,Proposals, the Board considered, among other factors, that the Proposals constitute part of a broader repositioning of the Fund intended to enable the Adviser to manage the Fund more effectively and in a manner more consistent with investor needs and reflective of the growth and structure of the bond market. In particular, the Board considered the Adviser’s statementstatements that the Adviser believes the Fund would benefit from changing its classification to non-diversifiedfundamental investment objective and fundamental investment policy because itthose changes would allow the Fund greater flexibility to hold larger positions inpursue a broader universe of investment opportunities, and that the securities of certain companies in whichchanges to the Fund’s portfolio managers have high conviction. The Board further considered the Adviser’s statement that, under current market conditions, non-diversification classificationfundamental investment objective and fundamental investment policies would be beneficial in executing the Adviser’s strategy for the Fund. The Board considered that a fund that invests a larger percentage of its assets in a smaller number of issuerslower rated securities could be more susceptible to negative events affecting those

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issuers, and in that regard, considered the Adviser’s belief that the potential benefits of investing more of the Fund’s assets in particular issuerslower rated securities as part of the Fund’s revised strategy justify the related risks, and noted the Adviser’s discussion of following a non-diversified strategy.portfolio risk management at the October meeting. Additionally, the Board considered that the costs associated with seeking shareholder approval of the ProposalProposals would not be borne by the shareholders given the expense limitation arrangements currently in effect with respect to the Fund.

THE BOARD OF TRUSTEES UNANIMOUSLY RECOMMEND THAT YOU VOTE IN

FAVOR OF THE PROPOSAL.PROPOSALS.

MISCELLANEOUS

Investment Adviser, Principal Underwriter and Administrator

WBIM and William Blair & Company, L.L.C. (“WBC”), each located at 150 North Riverside Plaza, Chicago, Illinois 60606, are respectively the Trust’s investment adviser and principal underwriter and

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distributor. Pursuant to a management agreement, the Adviser acts as the investment adviser to each series of the Trust, manages its investments, administers its business affairs, furnishes office facilities and equipment, provides clerical, bookkeeping and administrative services, provides shareholder and information services and permits any of its partners or employees to serve without compensation as trustees or officers of the Fund if elected to such positions. The Distributor acts as agent of the Trust in the sale of Fund shares. WBIM and WBC are collectively referred to herein as “William Blair.”

William Blair was founded over 80 years ago by William McCormick Blair. As of December 31, 2019,September 30, 2021, William Blair had over 1,5501,700 employees including approximately 180204 partners. WBIM oversees the assets of the Trust, along with corporate pension plans, endowments and foundations. As of December 31, 2019,September 30, 2021, WBIM managed over $58$77.4 billion in equities, fixed income securities, derivatives and cash equivalents. The Adviser is registered as an investment adviser under the Investment Advisers Act of 1940.

State Street Bank and Trust Company, located at One Lincoln Street, Boston, Massachusetts, 02111, provides custodian, valuation and certain administrative services to the Trust pursuant to various agreements.

Proxy Solicitation and Expenses

In addition to solicitations by mail, solicitations also may be made by telephone, through the Internet or in person by officers of the Trust or employees of the Adviser and by certain financial services firms and their representatives, who will receive no extra compensation for their services. Broadridge Financial Services, Inc. (“Broadridge”) has been engaged to assist in the solicitation of proxies for the Trust at an estimated cost of $55,000,$40,000, plus reimbursement for reasonable expenses. However, the exact cost will depend on the amount and types of services rendered. The cost of proxy solicitation will be borne by the Fund,Fund; however, by virtue of the operation of the expense limitation arrangements currently in effect with respect to the Fund, these costs will ultimately be borne by WBIM.

Shareholders may vote by mail, telephone or over the Internet. Shareholders who vote by telephone or over the Internet will have an opportunity to review their voting instructions and make

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any necessary changes before submitting their voting instructions and terminating their telephone call or Internet link. If a shareholder wishes to participate in the Meeting, but does not wish to give a proxy by telephone or over the Internet, the shareholder may still submit the proxy card originally sent with this proxy statement or attend the Meeting in person.by means of remote communication. Should shareholders require additional information regarding the proxy or a replacement a proxy card, they may contact Broadridge toll-free at 1-833-934-2740.

Adjournment

If a quorum is not present, the Meeting may be adjourned to a later date by the affirmative vote of a majority of the shares present. In the event that a quorum is present at the Meeting, but sufficient votes to approve the ProposalProposals are not received, the persons named as proxies may propose one or more adjournments of the Meeting to permit further solicitation of proxies. Any such adjournment must be approved by the affirmative vote of a majority of the shares present, in person or by proxy, at the Meeting. It is anticipated that the persons named as proxies would vote in favor of any adjournment. The Meeting may be adjourned without further notice to shareholders, but the Meeting may not be adjourned for more than six months beyond the originally scheduled meeting date.

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Proposals of Shareholders

The Trust does not generally hold annual shareholders’ meetings but will hold special meetings as required or deemed desirable. Because the Trust does not hold regular shareholders’ meetings, the anticipated date of the next shareholders’ meeting (if any) cannot be provided. Shareholders wishing to submit proposals for inclusion in a proxy statement for a subsequent shareholders’ meeting of the Trust, including submitting nominees for election as Trustees, should send their written proposals to the William Blair Funds, Secretary of the Trust at 150 North Riverside Plaza, Chicago, Illinois 60606. Proposals must be received in a reasonable time before the Trust begins to print and mail its proxy materials for the meeting. The timely submission of a proposal does not guarantee its inclusion.

Other Matters to Come Before the Meeting

The Board is not aware of any matters that will be presented for action at the Meeting other than the ProposalProposals set forth herein. Should any other matters requiring a vote of shareholders arise, the proxy in the accompanying form will confer upon the person or persons entitled to vote the shares represented by such proxy the discretionary authority to vote the shares with respect to any such other matters.

Householding Information

In order to reduce the amount of mail you receive and to help reduce Fund expenses, the Trust generally sends a single copy of any shareholder report and prospectus to each household unless instructed otherwise by a shareholder within the household. If you do not want the mailing of these documents to be combined with those for other members of your household, or if you wish to receive a single copy for your household, please call 1-800-742-7272 or submit your request in writing to the William Blair Funds at 150 North Riverside Plaza, Chicago, Illinois 60606.

A copy of the Trust’s most recent annual and semi-annual report is available without charge upon request by writing to the William Blair Funds, 150 North Riverside Plaza, Chicago,

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Illinois 60606 or by calling 1-800-742-7272. Reports are also available on the William Blair Funds website at www.williamblairfunds.com or at the website of the Securities and Exchange Commission at www.sec.gov.

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Please complete, sign, date and return the enclosed proxy card(s) (or take advantage of available telephonic or Internet voting procedures; see the proxy card for instructions) promptly. No postage is required if mailed in the United States.

Andrew T. Pfau

Secretary

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