Bell proposed a 10-week, $100,000 modification of two surplus Army Air Force P-63As; Grumman, an ugly modification of an F4F Wildcat, including a nose landing gear, and a more expensive, all-new design that included a second seat for a test engineer and the capability to be configured with both swept and straight wings.
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Bell had referred internally to the aircraft as the XP-63N but the Navy formally designated it L-39 on 19 April 1946. This was consistent with the Navy's practice for research aircraft, with the L being the letter assigned to Bell by the Navy and 39 being the Bell design number, as opposed to a model number. However, the number of at least one of Bell's L-39 flight test reports began with 33, which was the model number of the early P-63s. L-39-1 was assigned BuNo 90060 and L-39-2, BuNo 90061.