Even before the YP-38s had been built and delivered, on September 20, 1939 the Army placed an order for 66 P-38 fighters.
Twenty-nine of these were delivered as P-38-LO (company Model 222-62-02). The P-38 had the same powerplants as the YP-38, but armament was changed to one 37-mm cannon and four 0.50-in machine guns. Armor plate and bulletproof glass was added for pilot protection, and fluorescent instrument lighting was provided for night flying. Serials were 40-644/761 and 40-763/773.
P-38 Ser No 40-744 was later modified to study the effects on flight
crews of asymmetric cockpit location. The turbosuperchargers were
taken off the modified airplane, and the unit in the port boom was
replaced by a cockpit for a flight surgeon.