 The FR-1 Fireball was further developed into the XFR-2 which utilized a 1,425 hp (1,063 kW) Wright R-1820-74W in place of the -72W. One single airframe was converted to this configuration. No prototypes were built for the next proposed variant, the FR-3, which would have used a General Electric I-20 turbojet. Both of these projects were canceled with the end of the war. The fastest Fireball was the XFR-4, which had a Westinghouse J34 turbojet and was approximately 100 mph (161 km / h) faster than the FR-1. The turbojet's air intakes were moved from the wing roots to the fuselage in front of the wing; they were covered by electrically powered doors to lessen drag when the aircraft was flying only on its piston engine. The Fireball's fuselage was lengthened by 8 inches (203 mm) to accommodate the larger engine and the leading edge extension of the wing root that housed the air intakes was also removed. The XFR-4 was intended to serve as a testbed for the turbojet installation on the XF2R-1 Dark Shark. This was the final variant; the piston engine was replaced with a General Electric XT31-GE-2 turboprop, but only one prototype was built.
