Two war surplus P-63Cs were modified by Bell under Navy contract for flight testing of low-speed and stall characteristics of high-speed wing designs.
The aircraft received new wings with adjustable leading edge slats, trailing edge flaps and a pronounced sweep of 35 degrees.
The wings had no wheel wells, only the nose gear was retractable.
L-39-1 first flew 23 April 1946, demonstrating a need for extra tail surface and rear fuselage length to balance the aircraft in flight, the wing repositioning reduced empennage effectiveness and moved the center of lift aft.
A lighter three-bladed propeller from a P-39Q-10 was mounted and the necessary changes to the empennage were made.
L-39-2 incorporated these adjustments from the start.
L-39-1 later went to NACA at Langley for wind tunnel testing, where much valuable data were gathered.
L-39-2 also served as a test bed for the Bell X-2 40-degree wing design.