By 1943, the US Army Air Forces were bombing Japan with long-range B-29 bombers, but they (and the US Navy) lacked a suitable escort fighter with range to accompany the bombers. The P-51 Mustang had proven itself in the skies over Europe, and the classified Project Seahorse sought to test the feasibility of flying Mustangs from US Navy carriers sailing closer to Japan. A single P-51 was modified for carrier operations by strengthening the landing gear and fuselage and adding an arrestor hook. US Navy test pilot Lieutenant Robert M. Elder made 25 landings and takeoffs on the carrier USS Shangri-La with the redesignated ETF-51D* and, though the maritime Mustang proved capable of fulfilling the role, the capture of the islands of Okinawa and Iwo Jima in early 1945 removed the need for the Mustang as an escort in the Pacific and the project was shelved.
*Sources differ on whether or not the modified Mustang was actually redesignated ETF-51. The prefix change from P (Pursuit) to F (Fighter) did not occur until 1948, so the designation ETF-51 may have been applied after the war, long after the carrier testing program had finished.
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