Boeing XP-4

The Boeing XP-4 was a prototype American biplane fighter of the 1920s that was grounded permanently after just 4.5 hours of flight testing. In 1926 the US Army was very interested in the turbo-supercharger as a way of improving engine performance and asked that one be added to the last of the PW-9 fighters, whose engine was upgraded to a 510 hp Packard 1A-1500; the result was designated XP-4. Two extra .30 calibre guns were mounted under the lower wing, outside the propeller arc, and the lower wing span was extended by 9.5 feet to carry the added weight. Delivered to Wright Field for testing on 27 July 1927, it soon proved that the Packard engine could not make up for roughly 800 lb of extra weight, performed worse than its predecessor and the project was abandoned.