In 1927, William Boeing founded Boeing Air Transport to operate air mail routes under contract with the United States Post Office Department. In 1929, Boeing merged his company with Pratt & Whitney to form the United Aircraft and Transport Corporation (UATC) which then set about buying, in the space of just 28 months, Pacific Air Transport, Stout Air Services, VAL, and National Air Transport, as well as numerous equipment manufacturers at the same time. On March 28, 1931, UATC formed United Air Lines, Inc. as a holding company for its airline subsidiaries.