The XB-15 was a larger version of the Boeing B-17 Flying Fortress and had a wing span of 149 ft. (45.5 m). Construction was approved in February 1934 and its first flight was 15 October 1937.¹ At the time, it was the largest plane in the United States and had internal wing passages for inflight engine maintenance.² The XB-15 was always considered experimental and was grossly underpowered. The 850 hp Pratt & Whitney R-1830 Twin Wasps produced a disappointingly top speed of only 195 mph (314 km/h). Only one airplane was produced and afterwards, it converted to a transport with cargo doors and was relegated to cargo duties and redesignated as the XC-105.