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 centre of lift aft.
A lighter three-bladed propeller from a P-39Q-10 was mounted and the necessary changes to the empennage were made.
Incorporated these adjustments from the start.
L-39-2 also served as a test bed for the Bell X-2 40-degree wing design.
Later went to NACA at Langley for wind tunnel testing, where much valuable data were gathered.