The XP-72 was a development of the P-47 Thunderbolt. It was powered by the Pratt & Whitney R-4360-13 Wasp Major, a 28 cylinder radial engine capable of producing 3,500 hp. This was the most powerful piston engine to enter production during the Second World War. The majority of visual changes to the aircraft were as a result of this new engine. Despite the increase in size of the engine, the XP-72 had a slimmer nose than the P-47. This was achieved partly by moving the supercharger intake from the nose to the wing and partly by using a fan to cool the engine, allowing the use of a tight fitting engine cowling.
Republic received an order to build two prototype XP-72s on 18 June 1943. The first prototype flew on 2 February 1944. A second prototype, using a contra rotating propeller soon followed. Tests revealed that the XP-72 had the expected improvement in performance, with a top speed of 504mph. Republic received an order for 100 P-72s, but it was cancelled before production could begin.