In spite of the loss of the XP-38, the Lightning had shown its true potential. On April 27, 1939 a Limited Procurement Order for thirteen YP-38 service test aircraft was issued. Army serials were 39-689/701. The company designation for the planes was Model 122-62-02.
The YP-38 was redesigned for production and had a pair of 1150 hp Allison V-1710-27 and -29 (F2R and F2L) engines equipped with B-2 turbosuperchargers. These engines were equipped with spur reduction gearing rather than the former epicyclic type of gearing. This caused the engine's thrust line to be raised upward. The propellers were outward-rotating rather than inward-rotating as on the XP-38 (that is, the port propeller turned counterclockwise when seen from the rear and the starboard propeller turned clockwise).
The chin-mounted lip intake under the propeller spinner was replaced by a pair of cooling intakes. Enlarged coolant radiators were adopted on both sides of the tail booms.
Armament was revised to substitute two 0.30-in machine guns for two of the four 0.50-in machine guns, and a 37-mm Browning M9 cannon with 15 rounds was substituted for the 20-mm weapon. The 0.50 inch guns carried 200 rounds per gun and the 0.30 inch guns carried 500 rounds per gun. All the guns were mounted in the nose, with the 0.50 inch guns mounted above the 0.30-inch guns. One or two YP-38s were seen with prominent gun enclosure tubes protecting the two 0.50-inch machine guns, with flush plates covering the other gun ports. In reality, most YP-38s were flown without guns installed. At 14,348 lbs, the YP-38 was lighter than the overweight XP-38 due to structural redesign.
The first YP-38 flew on September 16, 1940 with Marshall Headle at the controls. In March 1940, the Army received its first YP-38 for service trials. Production lagged seriously behind schedule, and all thirteen YP-38s had not been completed until June of 1941. Maximum speed was 405 mph at 10,000 feet, and an altitude of 20,000 feet could be reached in six minutes. Normal range was 650 miles. Empty weight was 11,171 lbs, gross weight was 13,500 lbs, and maximum takeoff weight was 14,348 lbs.
During trials, the YP-38s ran into a problem in which the tail began to buffet severely during high speed dives, making it difficult to pull out. On November 4, 1941, the tail booms of YP-38 39-689 came off during a high speed dive over Glendale, California. Test pilot Ralph Virden was killed. This was initially falsely diagnosed as elevator flutter, and a set of external mass balances were added above and below the elevator. This problem was later solved by adding large wing-root fillets at the points where the wings joined the fuselage. This filleting had to be done very carefully, since failure to ensure a tight fit could severely impair the flight characteristics