The Curtiss Model 66 Swift was an unsuccessful competitor against the Boeing XP-936/P-26 for the US Army's interim monoplane pursuit of 1932. Encouraged by the Army, Curtiss undertook the development of a new pursuit as a private venture for which the Army agreed to provide the powerplant and the military equipment under a bailment contract. The experimental project number XP-934 was assigned.
The all-metal Swift drew heavily on the Curtiss A-8 Shrike attack plane. Like the P-26, the Curtiss Swift was an intriguing mixture of the old and the new. It was fitted with a low-mounted monoplane wing with external bracing struts. The fixed, non-retractable undercarriage was enclosed by a set of spats. The pilot's cockpit was fully enclosed by a sliding canopy. The wing was fitted with trailing-edge flaps and carried a set of full-span leading-edge slats that opened automatically at 15 mph above stalling speed. Armament was four 0.30-cal machine guns, two in troughs in the nose and two in external packages on each side of the cockpit.
The Swift had originally been planned for the 600 hp Curtiss Conqueror liquid-cooled V-12 engine, but the Army believed that this engine was nearing the end of its development cycle and insisted that the powerplant be changed to the 700 hp Wright R-1820 Cyclone air cooled radial. The aircraft was also fitted briefly with a Pratt & Whitney Wasp radial.
The Swift left the factory in July 1932. During the early flight testing, the performance was found to be rather disappointing. Within a month, the Cyclone radial was replaced by a Curtiss G1V-1570F Conqueror of 600 hp, which was the engine that Curtiss had wanted all along. Although the speed increased, other performance characteristics suffered because the plane was now seriously overweight. Maximum speed was 215 mph at sea level, initial climb rate was 2130 ft/min, service ceiling was 22,700 feet, and range was 396 miles. Weights were 3334 lbs. empty, 4143 lbs. gross.
The Army bought the XP-934 in February 1933 and assigned it the designation XP-31 and the serial number 33-178. The civil-type engine was replaced by an equivalent military-type V-1570-53 engine. The XP-31 was re-designated ZXP-31 (Z for obsolete) and was retired to an Air Corps mechanics' school in July 1936. It was surveyed at Edgewood Arsenal on December 10, 1936.