Curtiss XP-31 Swift

The Curtiss XP-31 Swift was an experimental single-seat monoplane fighter built privately by Curtiss and evaluated by the U.S. Army Air Corps in the early 1930s. First flown in July 1932 as the company-owned XP-934, it was purchased by the Army and designated XP-31 in March 1933. The Swift was a notably advanced design for its day: a low-wing, all-metal aircraft with the first enclosed cockpit on a U.S. Army fighter, trailing-edge flaps, and full-span leading-edge slots.

It was powered by a 600-horsepower Curtiss V-1570 Conqueror liquid-cooled engine and armed with four .30-caliber machine guns. Despite these innovations, the fixed landing gear and modest performance left it unable to outperform its rival, the Boeing P-26 Peashooter, and only the single prototype was built. The XP-31 nonetheless illustrated the rapid transition from biplane to monoplane fighter design.