Taxi tests of the XP-56 began on 6 April 1943 and showed a serious yaw problem. At first, it was thought to be caused by uneven wheel brakes, and considerable effort was placed into fixing this problem. Manual hydraulic brakes were installed and the aircraft flew on 30 September 1943 at Muroc Air Base in southern California. Eventually, the yaw problem was traced to a lack of aerodynamic stability, and to fix this the upper vertical stabilizer was enlarged from a mere stub, to one virtually matching the ventral unit in shape and area.
After a number of flights, the first XP-56 was destroyed 8 October 1943 when the tire on the left gear blew out during a high-speed (~130 mph) taxi across Muroc Dry Lake.
A number of changes were made to the second prototype, including re-ballasting to move the center-of-gravity forward, increasing the size of the upper vertical tail, and reworking the rudder control linkages. This second prototype was not completed until January 1944. The aircraft flew on 23 March 1944. The pilot had difficulty lifting the nose wheel below 160 mph (257 km/h). He also reported extreme yaw sensitivity. This flight lasted less than eight minutes, but subsequent flights were longer, and the nose heaviness disappeared when the landing gear was retracted. Only relatively low speeds were attained, however. While urging NACA to investigate the inability to attain designed speeds, further flight tests were made. On the 10th flight, the pilot noted extreme tail heaviness, lack of power, and excessive fuel consumption. Flight testing was then ceased as too hazardous, and the project was abandoned after a year of inactivity. By 1946, the U.S. Army Air Forces was developing jet-powered fighters, and had no need for a new propeller-driven fighter aircraft.