The P-63 participated in postwar swept-wing aerodynamic research. With the surrender of the Third Reich, the results of wartime German aerodynamic research became accessible to American military officials. This research indicated that the swept-back wing showed the most promise in making it possible for aircraft to achieve significantly higher speeds. Postwar research in the USA carried these ideas still further. The Navy was given the responsibility of investigating the low-speed stability behavior and stalling characteristics of the swept wing. The Navy contracted with Bell for the modification of two P-63C-5s which would be fitted with a pair of wings swept back at 35 degrees, with a short inboard straight section. The wings carried adjustable leading edge slats and trailing-edge flaps. All armament was deleted and the rear canopy was faired over. These two planes were redesignated L-39-1 and L-39-2 respectively. The rather odd designation for a Navy aircraft was gotten by using the Navy's code letter for Bell (L) and the company's model number (39). Tufts of string were attached to the sweptback wings in order to ascertain the airflow pattern, and a pair of movie cameras were mounted externally behind the cockpit canopy to film the airflow pattern while in flight.
The L-39-1 flew for the first time on April 23, 1946. After the first flight, a ventral fin was added underneath the rear fuselage to improve the stability. Early flight tests of the L-39-1 indicated that the center of gravity was too far forward, so a four-foot plug was added to the rear fuselage, and the four-bladed propellers were replaced by lighter three-bladed units taken from a pair of surplus P-39Q-10s. The L-39-2 was provided with the four-foot plug from the start, and had an even larger ventral fin.
Both aircraft tested a series of leading edge slat configurations of different designs. L-39-1 went to NACA for continuing flight tests, whereas L-39-2 remained at Bell. L-39-2 was later fitted with a completely swept wing of a design planned for the X-2 experimental rocket-powered research aircraft.