Boeing XP-4

The Boeing XP-4 was an experimental American biplane fighter of the late 1920s, developed from the company's successful PW-9 series. In 1926 the U.S. Army Air Corps, eager to exploit the turbo-supercharger for high-altitude performance, had Boeing rebuild the last PW-9 with a supercharged 510 hp Packard 1A-1500 engine. To carry the added weight and a heavier armament of four machine guns, the lower wing span was extended. Delivered to Wright Field on 27 July 1927, the XP-4 proved disappointing: the Packard engine could not overcome roughly 800 lb of extra weight, and the aircraft handled worse than the PW-9 it was meant to improve upon.

After only about 4.5 hours of flight testing it was permanently grounded, and the project was abandoned. The XP-4 nonetheless illustrates the Army's early interwar drive to extract greater altitude performance from existing pursuit designs.