Though rapidly overtaken in the U.S. airline market by the Douglas DC-2 and DC-3, the Boeing Model 247 still deserves recognition as having been the most advanced passenger aircraft in the world at the time it flew in February 1933. Besides its modern aerodynamic design, innovations such as all- aluminum, semi-monocoque construction, fully cantilevered wings, and retractable landing gear established a new standard for all commercial aircraft that followed it. Of 75 247s and 247Ds built, the USAAF impressed 27 (most from United Airlines) in 1942 under the designation C-73. The designation makes no apparent distinction between the earlier Model 247, recognizable by a forward- swept windscreen and ring cowls, and the 247D, which differed in having a swept- back windscreen, NACA cowls, and geared engines driving three- bladed, variable pitch propellers. During their wartime service, many of the older C-73s were re-powered with 600-hp R-1340-AN1 engines. Most had been returned to the airlines even before the war ended.
E.R. Johnson: American Military Transport Aircraft Since 1925