The Hughes Aircraft XP-73 is a mysterious aircraft; many authors maintain that it never existed and the P-73 designation was never used.
Constructed from Duramold, the Hughes D-2 was developed by Howard Hughes as a high-speed, long-range aircraft powered by two Wright Tornado engines, and with a configuration similar to that of the P-38 Lightning.
Hughes carried out testing of the D-2 at Harper Dry Lake in the Mojave Desert. The first flight was on 20 June 1943, and the aircraft was found to be unsatisfactory. Work than began to modify the aircraft, especially the wings; the modified aircraft was designated as the Hughes D-5.
It appears that the XA-37 designation may refer to the same aircraft.