The fastest version of the Thunderbolt was the XP-47J, which was proposed in November 1942 as a lighter-weight version of the Thunderbolt designed to explore the outer limits of the design's basic performance envelope. The XP-47J was fitted with a 2800 hp Pratt and Whitney R-2800-57(C) housed inside a close-fitting cowling and cooled by a fan. The ventral intake for the CH-5 turbosupercharger was separated from the engine cowling and moved aft. The four-bladed propeller was fitted with a large conical-shaped spinner. The wing structure was lightened and the armament was reduced from eight to six 0.50-inch machine guns. The contract was approved on June 18, 1943.
The XP-47J was a completely new airframe and not a conversion of an existing P-47D. The serial number was 43-46952. The XP-47J flew for the first time on November 26, 1943. On August 4, 1944, it attained a speed of 504 mph in level fight, becoming the first propeller-driven fighter to exceed 500 mph. At one time, it was proposed that the J model would be introduced onto the production line, but the advent of the even more advanced XP-72 resulted in plans for the production of the P-47J being abandoned before any more could be completed.
A proposal to adapt the XP-47J to use contrarotating propellers with an R-2800-61 engine was dropped.
Maximum speed of the XP-47J was 507 mph at 34,300 feet, range was 765 miles at 400 mph, 1070 miles at economical cruising speed. An altitude of 15,000 feet could be reached in 4.5 minutes. Service ceiling was 45,000 feet. Weights were 9663 pounds empty, 12,400 pounds normal loaded, 16,780 pounds maximum. Wingspan was 40 feet 11 inches, length was 33 feet 3 inches, height was 14 feet 2 inches, and wing area was 300 square feet.