To improve the aerodynamics of the air-cooled piston engines, the fourth production P-36A (serial number 38-004) was equipped with the long streamlined fairing of a liquid-cooled engine. Twelve different designs were tested with little success – although the aircraft was faster than a regular P-36A, the problems with cooling the engine could not be solved. Since the new P-40 was faster, the project was suspended. At the end of its service time the only XP-42 was given a fully movable tailplane to study this configuration of the elevator.
Peter M. Bowers: Forgotten Fighters and Experimental Aircraft U.S. Army 1918-1941 2