The XP-31 was the first monoplane pursuit aircraft built by Curtiss and was designed to compete with the Boeing P-26. The XP-31 (S/N 33-178) was tested as the XP-934 and initially had a Wright R-1820 Cyclone radial engine installed. Because of poor performance, the engine was replaced with a Curtiss Conqueror, which radically altered its appearance, but didn't improve performance enough to warrant production.
The XP-31 featured the first enclosed cockpit on a pursuit aircraft (it was built before the P-29). It was also the last pursuit aircraft to have fixed landing gear and externally braced wings.
First with radial, then "vee" engine
TECHNICAL NOTES (with V-1570 engine): Armament: Two .30-cal. machine guns and (later) two .30-cal. machine guns in blisters mounted beside the cockpit Engine:Curtiss V-1570-53 Conqueror of 600 hp Maximum speed: 208 mph Service ceiling: 24,400 ft. Range: 370 mi. Weight: 3,334 lbs. empty/4,143 lbs. (maximum) Span: 36 ft. 0 in. Length: 26 ft. 3 in. Height: 7 ft. 9 in.