Which American fighter was in large-scale production throughout US participation in World War II?

Lockheed designed the P-38 to meet a February 1937 specification from the Army Air Corps (USAAC), Circular Proposal X-608. It won the competition on June 23, 1937, with its Model 22, and the XP-38 prototype first flew on January 27, 1939, piloted by Ben Kelsey. On September 20, 1939, even before the pre-production YP-38s had been built and tested, the Air Corps ordered 66 initial production P-38s, 30 of which were delivered by mid-1941.

When the United States entered the war, the Lightning was already on the production lines, and it never left them until the end: it was the only American fighter manufactured on a large scale from Pearl Harbor to the day of victory over Japan. The P-38J-25-LO block and the later P-38L are considered the definitive Lightnings; Lockheed sped up production with subcontractors across the country to build hundreds of aircraft a month. The P-38L was the most numerous variant, with 3,923 units, 113 of them assembled by Consolidated-Vultee at its Nashville plant.

This continuity was reflected in combat. In the Pacific, the P-38 shot down more than 1,800 Japanese aircraft and was the aircraft of the country's top aces, Richard Bong and Thomas McGuire. In Europe, however, the smaller, more aerodynamic P-51, faster in a dive, replaced it in most fighter groups by mid-1944.

It was also the first military aircraft to exceed 400 mph (640 km/h) in level flight and the first American fighter to make extensive use of stainless steel and smooth, flush-riveted aluminum panels.