The first XFR-1 made its first flight on 25 June 1944 without its jet engine, but this was installed shortly afterward. The second prototype first flew on 20 September 1944. Test flights confirmed wind tunnel tests that revealed a lack of longitudinal stability because the center of gravity had been miscalculated. In addition, the circular rear fuselage of the FR-1 gave less stability than the slab-style fuselage of the Grumman F4F Wildcat that was used as a model for the stability calculations. A new tail with enlarged vertical and horizontal stabilizers was designed and retrofitted to the prototypes. The original Douglas double-slotted flaps proved to be unsatisfactory during flight testing, but all three prototypes and the first 14 production aircraft were built with them before they were replaced with a single-slotted flap. The first prototype was lost in a crash at NAS China Lake on 13 October 1944. Investigation showed that the wing structure was not strong enough to resist compressibility effects. This was cured by doubling the number of rivets in the outer wing panels. The second prototype crashed on 25 March 1945 when the pilot failed to recover from a dive from , probably also due to compressibility effects. The third prototype crashed on 5 April when the canopy blew off during a high-speed pass over Lindbergh Field. Operational testing by the Naval Air Test Center at Naval Air Station Patuxent River that included carrier acceptability tests revealed additional problems: The piston engine tended to overheat until electrically operated cowl flaps were installed, the catapult hooks had to be moved, and the nosewheel oleo shock strut had to be lengthened by . Carrier suitability tests began aboard the escort carrier in early January 1945. The aircraft successfully made five catapult takeoffs using the piston engine as well as three takeoffs using both engines. No problems were reported when landing aboard the carrier. 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 (160 km/h) faster than the FR-1.