Lockheed YP-38
While the pre-production YP-38 models shared a strong resemblance to the prototype, several subtle differences emerged. The aircraft incorporated a modified engine with a B-2 supercharger and a spur reduction gear, necessitating adjustments to the engine oil line. The distinctive cowling design of the prototype was replaced by a wider configuration with twin elliptical cooling intakes, aligning more closely with subsequent Lightning models. The radiator intakes on the tail booms were also expanded to accommodate the turbocharger exhausts.
Originally equipped with four .50 caliber machine guns, the armament was revised to include two .30 caliber and two .50 caliber machine guns, along with a 37mm cannon. These weapons were integrated during extensive service testing conducted by the Army. One YP-38 was transferred to the National Advisory Committee for Aeronautics (NACA) for evaluation from November 27, 1941, to February 4, 1942.