John M. Duff Jr. has been elected senior vice president for direct credits and financial guarantees of the Export-Import Bank of the United States, succeeding recently retired Edward J. O'Conner.

Duff will administer Ex-Im Bank's long-term lending and guarantee programs, which support U.S. exports of heavy capital equipment and domestic contracts for major foreign construction contracts.

A part-time professor at the Duquensne Law School, Duff is one of the Ex-Im Bank's principal negotiationrs working to strenghten the International Arrangement on Exports Credits among the export credit agencies of the world's major trading nations. He was previously the bank's deputy general counsel.

Frank W. Yount has been elected president of the new Frank S. Philips Sales Corp., part of the Frank S. Phillips Companies, active in area development, sales and insurance since 1933. Yount has been with Phillips since 1947, specializing in real estate sales and property management.

Paul P. Massa has been appointed exectutive vice president and chief operating officer of Congressional Information Service Inc. of Bethesda. He was formely vice president and general manager of Congressional Quarterly Inc.

Two new directors have been added to the board of the National Bank of Washington. They are Gary M. Callen, a consultant at the United Mineworkers of America, and William M. Cafritz, president of a Bethesday corporation bearing his name.

Thomas R. Harter has been named senior staff vice president of the Mortgage Bankers Association of America.

Richard L. Dunham has been elected president of H. Zinder & Associates, a division of Zinder Compainies. He is a former chairman of the Federal Power Commission.