The first of two prototype XF-91 single-seat supersonic interceptors powered by a J47 and a 6000 lbs XLR11 rocket. First flew 9 May 1949. No serial production of the type was undertaken.

While developing the XP-84, Republic looked at installing rockets in fighters, inspired by late wartime German research. However the resulting XP-91 was quickly obsolete thanks to the rapidly increasing performance of newer jet engines and only two prototypes were built, although one was the first US fighter to exceed Mach 1 in level flight. An unusual feature was the wing planform, markedly tapered outwards from the fuselage to a wide chord at the wingtips to combat stall problems with early swept-wing designs