The XB-15 was designed around
liquid-cooled 1,000 hp (750 kW) engines. Unfortunately, these were not available, and 850 hp (637 kW) radial air-cooled engines were used instead. These engines left the bomber significantly underpowered; its top speed of 200 mph (322 km/h) was far too slow for a combat aircraft, and the project was abandoned. Even without the improved defensive armament that would have been needed in service, the XB-15 had a maximum takeoff weight 5,000 pounds greater than the later
B-17G, but with a total engine output of 1,800 less horsepower.
No B-15s were put into service; the Army Air Corps converted the only prototype into a
transport designated the
XC-105, which carried freight around the
Caribbean during
World War II. The sole XC-105 was scrapped in
Panama in 1945.