Robert Stanley had always been interested in Vee-shaped tails, ever since the 1930s. During the last few months of World War II, Stanley convinced the Army to let him try out his ideas for Vee tails on a P-63.
In support of this program, P-63A-9 42-64606 was experimentally fitted with a Vee tail. In order to provide for additional stability, extra keel area was added in the form of a ventral fin. Several Bell test pilots got a chance to fly the ship, including Stanley himself. The results were not all that encouraging-some longitudinal instability was encountered at low speeds and "Tex" Johnston noted some unusual spinning characteristics. The project was duly dropped.
After the war, the Air Force exhibited some renewed interest in Vee tails for high speed jet aircraft. In 1948 the USAF issued an order for the conversion of RP-63G 45-57300 to a Vee-tailed aircraft. Some sources suggest that the designation XP-63N was applied to this aircraft. The aircraft tested butterfly tails of 38 and 43 degrees, and did not have the ventral fin of the earlier modified P-63A. An air data boom was mounted underneath the starboard wing. Test flights indicated that there were no major spinning or stability problems, but crosswind characteristics were rather poor. The program was once again dropped.