The Republic XP-72 "Super Thunderbolt" was an American prototype interceptor fighter developed as a progression of the P-47 design. The XP-72 was designed around the Pratt & Whitney R-4360 twenty-eight-cylinder air-cooled radial engine with a supercharger mounted behind the pilot and driven by an extension shaft from the engine. The armament consisted of six 0.5 in wing-mounted machine guns and underwing racks for two 1,000 lb bombs.
The XP-72 development paralleled that of another Republic design, the XP-69 that was to be powered by an experimental forty-two-cylinder Wright R-2160 liquid-cooled radial engine mounted behind the pilot and driving contra-rotating propellers through an extension shaft. The XP-69 was intended for high altitude operations and featured a pressurized cockpit and armament of two 37 mm cannon and four 0.5 in machine guns. As the XP-72 displayed greater promise than the XP-69, the XP-69 was canceled on 11 May 1943 and an order for two XP-72 prototypes was placed on 18 June 1943.
The XP-72 flew for the first time on 2 February 1944, equipped with a four-bladed propeller. The second prototype was completed on 26 June 1944 and was equipped with an Aero-Products contra-rotating propeller. As the XP-72 displayed exceptional performance during flight tests, an order for 100 production aircraft was awarded. The order included an alternate armament configuration of four 37 mm cannon. By this time the war had progressed to where the need was for long-range escort fighters and not high-speed interceptors. Also, the advent of the new turbojet-powered interceptors showed greater promise for the interceptor role. Thus, the production order for the P-72 was cancelled.