In mid-1942, Republic Aviation Corporation initiated a design study to lighten their P-47 Thunderbolt fighter for improved performance. The Thunderbolt had been steadily gaining weight as the design matured, while comparative enemy aircraft, like the Focke-Wulf FW 190A, were much lighter. Republic officially proposed a light-P-47 to the Army Air Force (AAF) on 22 November 1942. On 1 April 1943, the AAF gave Republic a letter of intent to purchase two light-weight P-47s, and the contract was officially approved on 18 June 1943. This new aircraft was designated the XP-47J.

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The XP-47J was similar in appearance to a P-47B, but it was a completely new aircraft. The XP-47J had a close-fitting cowl installed around its Pratt & Whitney (P&W) R-2800 engine of increased power output. A large spinner was added, along with a fan to aid engine cooling. The turbosupercharger’s intake had been refined, and the flow of its exhaust was directed to provide additional thrust. Two of the .50-cal machine guns were deleted (leaving six) in the XP-47J’s lightened wing, and the rounds per gun were reduced to 267. Other weight-saving measures were the omission of radio equipment and the aft fuel tank. To keep the aerodynamics clean, the XP-47J had no provisions for external stores under the wings or fuselage.

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On 31 July 1943, Republic issued a report comparing the estimated performance of the XP-47J with the P&W R-4360-powered XP-72 that was under development. The report concluded that the Republic XP-72 had more potential and recommended resources be focused on that aircraft. In addition, 70% of the P-47 production line needed to be re-tooled in order to produce the P-47J. Republic called for the cancellation of the second XP-47J prototype but encouraged the completion and testing of the first prototype. The AAF approved Republic’s plan and cancelled the second XP-47J.

The sole XP-47J prototype (serial number 43-46952) was completed in mid-November 1943 and made its first flight on the 26th of that month. The aircraft was quickly dubbed Superbolt and wore nose art on both sides of the cowling of Superman holding a lightning bolt. After about 10 hours of flying time, the R-2800-57 engine was making metal and was replaced by another engine of the same type in February 1944.

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While the XP-47J Superbolt had remarkable performance, it was overshadowed by other aircraft, like the XP-72, that were under development. Work on the XP-72, which first flew on 2 February 1944, was not far behind that of the XP-47J, but as the war progressed and with the advent of jet fighters, neither aircraft were needed.

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