The Republic XP-72 was Alexander Kartveli's proposal for a replacement for his fabulously successful P-47 Thunderbolt. It was evolved in parallel with the XP-69 escort fighter project.
The XP-69 was a completely new design, whereas the XP-72 was a more-or-less straightforward progressive development of the P-47 Thunderbolt. The XP-72 was the first fighter to be designed around the huge 28-cylinger Pratt & Whitney R-4360 Wasp Major, the most powerful piston engine produced during World War 2. The engine was close-cowled and was fan-cooled. The Wasp Major was to have driven a pair of three-bladed Aeroproducts contrarotating propellers. The wing and tail of the P-47D were to be retained, but the airframe was enlarged and strengthened. The turbosupercharger was still located aft of the cockpit as it was in the P-47, but the turbosupercharger intake was moved to a position just underneath the cockpit rather than in the extreme nose. The lower fuselage was modified to allow for the larger air intake for the turbosupercharger. The XP-72 was to have been fitted with a bubble-type canopy, similar to that used by the late production blocks of the P-47D. Compressibility recovery flaps were fitted. Armament was to consist of six wing-mounted 0.50-inch machine guns, and two 1000-pound bombs were to be carried on underwing shackles.
The greater promise displayed by the XP-72 caused the USAAF to cancel the XP-69 project in favor of the XP-72. Two XP-72 prototypes were ordered on June 18, 1943. Serial numbers were 43-36598 and 43-36599. Some sources have these serials as being 43-6598 and 43-6599, but these appear to be in error since they conflict with a batch of P-51 Mustang serials.
XP-72 Ser No 43-36598 flew for the first time on February 2, 1944. Power was provided by a 3450 hp Pratt & Whitney R-4360-13 Wasp Major air-cooled radial engine. Since delivery of the planned Aeroproducts contrarotating six-bladed propeller had been delayed, the first XP-72 was equipped with a single four-bladed propeller as a temporary stop-gap measure. Nevertheless, the performance was excellent, a maximum speed of 490 mph being reached in flight tests. The second XP-72 (43-36599) flew for the first time on June 26, 1944. It was fitted with the Aeroproducts contrarotating propellers, which had finally been delivered. Unfortunately, the second XP-72 was written off in a takeoff crash early in its test flight program.
In spite of the loss of the second prototype, the USAAF was so impressed with the performance of the XP-72 that they ordered one hundred P-72 production variants. These P-72s were to have the R-4360-19 engine and were to be provided with an optional choice of four 37-mm cannon as an alternative for the six 0.50-in machine guns. It was anticipated that speeds in excess of 500 mph would be attained.
However, the changing character of the war created a greater need for long-range escort fighters than for high-speed interceptors, and the USAAF rapidly lost interest in the XP-72 project. The order for the one hundred P-72s was cancelled. The advent of jet-powered fighters which promised even more spectacular performance was undoubtedly also a factor. The surviving XP-72 (43-36598) is thought to have been scrapped at Wright Field at about the time of V-J Day.
Specification of Republic XP-72:
Powerplant: One 3450 hp Pratt & Whitney R-4360-13 Wasp Major air-cooled radial engine. Performance: Maximum speed was 490 mph at 25,000 feet. Normal range was 1200 miles at 300 mph and maximum range was 2520 miles at 315 mph with two 125 Imp. gall. drop tanks. Initial climb rate was 5280 feet per minute, and climb rate at 25,000 feet was 3550 feet per minute. An altitude of 15,000 feet could be reached in 3.5 minutes, 20,000 feet in 5 minutes. Service ceiling was 42,000 feet. Weights were 11,476 pounds empty, 14,433 pounds normal loaded, 17,490 pounds maximum. Dimensions were wingspan 40 feet 11 inches, length 36 feet 7 inches, height 16 feet 0 inches, and wing area 300 square feet.