<PAGE>

                                                                  EXHIBIT 10(AA)


                       [BANK ONE CORPORATION letterhead]


                                                        March 1, 2000


VIA FEDERAL EXPRESS
-------------------

Mr. William P. Boardman
Vice Chairman
BANK ONE CORPORATION

Dear Bill:

This letter will set forth the agreement between you and BANK ONE CORPORATION
(the "Corporation"), as approved by the Corporation's Board of Directors on
January 18, 2000 and February 15, 2000, with respect to certain post-retirement
benefits and stock entitlements. This letter will supercede all prior written or
oral agreements pertaining to your continued employment with the Corporation and
all of its subsidiaries (collectively, "Bank One") or the termination thereof,
including the letter agreements dated August 3, 1994 and December 18, 1999, but
excluding (a) the Change of Control Employment Agreement between you and the
Corporation effective October 1, 1999 (the "COC Agreement"), and (b) the
Executive Management Separation Plan (as augmented in the memorandum dated
February 16, 2000 from me to you and other members of the Management Committee),
under which plan you will be eligible for benefits only if you are not eligible
for retirement benefits under this agreement.

Bank One's Obligations
----------------------

If you remain actively employed by Bank One until the date you attain age 60, or
if your employment with Bank One terminates before that date (a) under
circumstances which would entitle you to termination benefits under the COC
Agreement, or (b) at the initiation of the Corporation for a reason other than
"cause," then upon your termination of employment:

     1)   You will be entitled to an aggregate retirement benefit from Bank One
          (from all sources, including the qualified and nonqualified pension
          plan, the supplemental executive retirement plan and the COC
          Agreement, if applicable) of $525,000 per year for your life, payable
          monthly beginning on the first day of the month following the date
          your employment terminates. This benefit will be subject to applicable
          withholding taxes and may be paid at your election in an
<PAGE>

                                                         Mr. William P. Boardman
                                                                   March 1, 2000
                                                                          Page 2


          actuarially equivalent amount in any form then available under the
          qualified pension plan, subject to completion of required election
          forms.

     2)   From the date of your termination of employment until the date you
          attain age 65 (or in the event of your death before age 65, the date
          your surviving spouse attains age 65), Bank One will reimburse you on
          an after-tax basis for the difference between the monthly contribution
          you pay (reduced by any post-retirement medical subsidy to which you
          are entitled) to maintain retiree medical coverage under the Bank One
          Medical Plan or any successor plan, and the amount you would have been
          required to pay for the same coverage as an active employee.

     3)   All stock options of the Corporation outstanding to you as of January
          18, 2000 will become fully exercisable on the date your employment
          terminates and will remain exercisable for a period of ten years after
          that date, or until the option expiration date, if earlier. The
          restrictions on all restricted stock of the Corporation outstanding to
          you as of January 18, 2000 will lapse on the date your employment
          terminates. Any ordinary annual awards of stock options or restricted
          stock made to you after January 18, 2000 will provide by their terms
          for full vesting and ten-year post-retirement exercise periods, in the
          case of options, and the lapse of all restrictions, in the case of
          restricted shares, upon your retirement at or after age 60, or before
          age 60 in the event of a change of control or a Corporation-initiated
          termination as described above. (These provisions shall not be
          included in any special or extraordinary stock awards, such as the
          special retention stock options covering 147,300 shares granted to you
          on February 15, 2000.)

You will not be entitled to any of the foregoing benefits under this agreement
if you voluntarily resign or if your active employment (which does not include
any pay continuation or severance period) terminates before age 60 for any
reason other those set forth above in clauses (a) and (b) under "Bank One's
Obligations," above. "Cause" means a material violation of the Corporation's
code of ethics or gross negligence, willful misconduct, fraud or dishonesty, as
determined by the Organization, Compensation and Nominating Committee of the
Board of Directors of the Corporation.

Upon the sale of your house in Wilmington, (a) the Corporation will reimburse
you on an after-tax basis for the amount, if any, by which your capital
investment in the house (purchase price plus reasonable capital improvements)
exceeds the sale price on the house; or (b) you will pay the Corporation the
amount, if any, by which the sale price of the house exceeds your capital
investment in the house. Your anticipated capital investment in the house is
less than $800,000.
<PAGE>

                                                         Mr. William P. Boardman
                                                                   March 1, 2000
                                                                          Page 3


Following the termination of your employment, you will continue to be
indemnified, defended and held harmless against judgments, fines and reasonable
expenses, including attorney's fees incurred by you in any lawsuit or threatened
lawsuit in which you are involved by reason of the fact that you were an officer
or employee of the Corporation, to the maximum extent permitted by the
Corporation's by-laws and certificate of incorporation.

Your Obligations
----------------

In addition to the fiduciary and other obligations you will have as a former
officer and director of the Corporation, the following will apply upon
termination of your employment under circumstances which entitle you to the
retirement benefits as set forth above:

     1)   Following the termination of your employment with Bank One, you will
          continue to hold in a fiduciary capacity for the benefit of the
          Corporation all secret or confidential information, knowledge or data
          related to Bank One which is or was obtained by you during your
          employment with Bank One and which does not become public knowledge
          (other than by your acts or the acts of your representatives). You
          will not, without the prior written consent of the Corporation or as
          otherwise required by law, communicate or divulge such information to
          anyone other than the Corporation and those designated by it. In
          addition, you will not make any public statements, encourage others to
          make public statements or release any information intended to
          disparage the Corporation or any of its subsidiaries or any of their
          respective officers or directors.

     2)   For twelve months after the date your employment terminates, you will
          not directly or indirectly own, manage, operate, control or
          participate in the ownership, management, operation or control or be
          connected as an officer, director, consultant or otherwise with any
          business engaged in the financial services business which is in
          material competition with the businesses conducted by Bank One. This
          paragraph shall not prohibit you from engaging in the activities of an
          investment banker or consultant to an investment banking firm.

     3)   For twelve months after the date your employment terminates, you will
          not, directly or indirectly, on your own behalf or on behalf of any
          other person or entity solicit for employment or employ any person
          then employed by Bank One.

     4)   Following the termination of your employment, you will assist the
          Corporation in any investigation, litigation or threatened litigation
          related to any matter in which you were involved or of which you have
          knowledge as a result of your employment with Bank One. The
          Corporation will reimburse you for all
<PAGE>

                                                         Mr. William P. Boardman
                                                                   March 1, 2000
                                                                          Page 4

          reasonable out-of-pocket expenses associated with such assistance,
          including travel expenses.

This agreement will be binding upon and inure to the benefit of you and the
Corporation and your respective heirs, successors, and assigns.

Please indicate below your agreement with and acceptance of the foregoing terms,

                                         Sincerely,

                                         /s/

BANK ONE CORPORATION


By:  /s/ Timothy P. Moen
     --------------------------
     Timothy P. Moen
     Executive Vice President

As of 8th day of March, 2000, I have reviewed, am in full agreement with and
accept the foregoing agreement.



                                 /s/ William P. Boardman
                                 ---------------------------
                                        William P. Boardman

cc:  John R. Hall
     Verne G. Istock