1 x Pratt & Whitney R-2800-57C turbo, supercharged, air-cooled, radial piston engine developing 2,800 horsepower and driving a four-bladed propeller.
505 mph (813 kph; 439 kts)
44,997 feet (13,715 m; 8.52 miles)
1,072 miles (1,725 km; 931 nm)
4,900 ft/min (1,494 m/min)
6 x 0.50 calibre Browning heavy machine guns
A similar attempt to improve performance resulted in the XP-47J. Unlike the XP-47H, the sole XP-47J was a newly built airframe, with many changes to reduce weight. The aircraft was fitted with a 2,800 hp R-2800-57C engine with a CH-5 supercharger, housed in a redesigned streamlined cowling. Armament was reduced to six M2 Brownings.
The XP-47J was first flown on November 26, 1943, and on August 4, 1944, it became the first piston-engined fighter to exceed 500 mph, with a speed of 504 mph, making it the fastest Thunderbolt variant. A production version of the XP-47J was cancelled in favour of another Thunderbolt development, the XP-72, as were plans for installing an R-2800-61 engine with contra-rotating propellers.