Europe already had twin-engine fighters in 1937 with the Messerschmitt Bf 110 and Fokker G-l, the latter even anticipating the Lightning’s tail booms, but these ships were long-range two-seaters. Lockheed’s XP-38 was a single-seat fighter far outdoing the comparable Westland Whirlwind built in Britain in 1938.
On January 8, 1937, the Air Corps secretly invited five companies to participate in a competition to design a experimental fighter, whose mission was to intercept enemy long-range bombers. Since such bombers would be unlikely to be escorted by fighters, the defender’s speed, climb, and firepower would be more important than aerobatic and endurance potential.
The specification, X-608 Interceptor Pursuit (Twin-engine), called for a top speed of 360 to 400 mph at 20,000 feet and a climb to that altitude in six minutes. This performance required turbosupercharged Allison engines, and armament should include a cannon and four machine guns.
Lockheed’s experience with twin-engine designs, including the Electra transport and the proposed XFM-2, prepared the way for C.L. “Kelly” Johnson’s bold Model 22 design, which became the XP-38.
The engines were the same Allisons, rated at 1,150 hp for takeoff, used on the XP-37, but they had their propellers turning inward to counteract each other’s torque. Designated V-1710-11 and V-1710-15, they had General Electric B-1 turbosuperchargers in the twin booms that extended back to twin rudders. A short central nacelle held the pilot and proposed armament of one 23-mm Madsen and four .50-caliber Browning guns, which shot a concentrated stream of bullets from the nose uninterrupted by synchronization.
The first tricycle gear on a fighter permitted faster landings than were safe for older types, although Fowler extended flaps were needed to keep stalling speed within reason. Designer Johnson also provided a control wheel, instead of the usual stick, for the metal-covered control surfaces.
A wind-tunnel model was tested at the California Institute of Technology on April 1, 1937, a week after they had tested a model of the rival Vultee XP-1015. Using the same power plants and armament as the Model 22, the even larger Vultee had a long fuselage and conventional tail-down landing gear. Lockheed’s proposal was submitted to the Air Corps on April 13, along with the lowest price bid, compared to bids by Hughes, Vought, and Vultee. The XP-38 was ordered June 23, 1937, but it would take over five years before service versions could enter combat.
The original XP-38 specification called for a 417-mph top speed at 20,000 feet (which was to be reached in 4.5 minutes), a service ceiling of 39,100 feet, and an endurance of 1.75 hours at 393 mph. This was promised at weights of 7802 pounds empty and 10,500 pounds gross.
By the time of the XP-38’s first flight by Lt. Ben Kelsey on January 27, 1939, 3,700 pounds had been added to that weight, and Lockheed had spent $761,000 on a $163,000 contract. It was a good investment, for despite flap trouble on its first flight, the XP-38 was clearly the most advanced anti-bomber weapon available in the world. Lockheed had reduced its top speed guarantee to a conservative 385 mph with a 14,200 pounds gross weight, but this was soon surpassed in tests at March Field.
The secrecy that had surrounded the project was lifted for a transcontinental speed dash on February 11, 1939. Although the flight ended in a crash landing that destroyed the prototype, thirteen YP-38s (Model 122) were ordered on April 27.
Sixty-six Model 222 production aircraft were added by a contract announced August 10 and approved September 16, 1939. These aircraft used Allison F-2s (V-1710-27/29) with outward turning propellers and whose short gearbox and higher thrust line gave a new shape to the engine nacelles. They were supposed to be delivered in 15 months, so the First Pursuit Group at Selfridge Field could replace its P-35 and P-36s by December 30, 1940.
When the Anglo-French Purchasing commission was allowed to buy the most advanced American fighters, Secretary of War Harry Woodring objected to the export of turbosuperchargers. An isolationist at heart, he felt them too secret and too scarce to share. While the mechanical principles were no secret, it had taken 20 years to develop the metallurgical technology for the Air Corps. On March 21, 1940, Lockheed offered an export version, Model 322-61 with two 1,090-hp V-1710-C15s, without a turbosupercharger, turning both propellers in the same direction. Since these were the same engines used by the P-40 Tomahawks, they presented an attractive alternative for the Allies.
On paper, the threat to the type’s performance was not fully realized. Guaranteed YP-38 performance at the design weight of 13,500 pounds promised 353 mph at 5,000 feet, 405 mph at 20,000 feet, and a climb to 20,000 feet in six minutes, while the 322-61 without turbos was expected to do 361 mph at 5,000 feet, 404 mph at 16,000 feet, and climb to 16,000 feet in 5.6 minutes.