On 17 May, 1946, Bell Aircraft announced that 'experimentation with the use of sweptback wings in an effort to reach the ultimate goal of man-carrying flight beyond the speeds of sound has passed from the high-speed wind-tunnel stage to the use of full-scale aircraft'.

This announcement marked the first stage development of the US Navy research swept-back wing experimental programme. To do so, two P-63 airframes were diverted for modification: one was an XP-63N with 379 hours of flight time and received the model designation L-39-1; the other was a P-63A-9-BE and received the model designation L-39-2. The main modification consisted of mounting P-63E-like outer wing panels with 35 degrees sweep-back. The main undercarriage was not retractable but the nose wheel was still retractable. To modify the centre of gravity position, ballast was installed in the rear fuselage. But it was later discovered that this ballast was insufficient and it was decided to change the four-blade propeller for a lighter three-blade one (from a P-39Q). After further tests, the fuselage was lengthened by four feet, a large ventral fin was added and wing slots installed in the wing leading-edges.

The maiden flight of the L-39-1 was made on 23 April, 1946, at Niagara Falls Airport with A M 'Tex' Johnston, Bell test pilot, at the controls. After several other test flights, the aircraft was ferried to NACA Langley Research Center on 22 August, 1946, by L W Grey. Meanwhile the L-39-2 had been flown by Johnston. This aeroplane was equipped with an automatic fuel equalizer designed to maintain a constant position of centre of gravity during flight. Both aircraft were used to correlate previous wind-tunnel experiments and were also used for various other experiments. For example, in August 1946, L-39-2 was fitted with a circular arc aerofoil developed for the future Grumman XS-2F Tracker.

All tests were halted on 26 August, 1946, and the entire programme was withdrawn. L-39-2 was sent to Langley on 11 December, 1946, and both aircraft were transferred to NACA Lewis Research Center in Cleveland, Ohio, on 12 December, 1949. The two airframes were eventually sold as scrap in 1955.

Alain J Pelletier "Bell Aircraft since 1935", 1992