The 1930s and 40s saw a great many unusual aircraft designs appear. However, very few of those designed produced a successful combat aircraft. Perhaps the most successful of these innovative designs was the Lockheed P-38 Lighting, a twin boomed fighter that for most of the war was the best long range fighter available to the USAAF, before being largely replaced by the P-51 Mustang. Over the vast expanses of the Pacific, the extra safety provided by its twin engined design meant that it remained a favourite aircraft until the end of the war.
Despite initial concerns, the big twin engined fighter proved able to more than hold its own against both German and Japanese opposition. America’s two highest scoring air aces of the Second World War, Richard Bong and Thomas McGuire, both flew the P-38.
The XP-38 was designed to satisfy the requirements of Army specification X-608, issued in late 1935. This called for a fighter with a top speed of 360 mph at 20,000 feet, capable of maintaining that speed for an hour, and armed with at least four machine guns (double the number standard in earlier American fighter aircraft).
Lockheed responded with a twin engined aircraft of unusual design. The two engines (Allison V-1710 “C” series) would be carried in streamlined nacelles that then tapered back into twin booms, each ending in a vertical tail plane, and connected by a horizontal stabilizer. The pilot would be carried in a small pod carried between the engines. The guns would be concentrated in the nose of that central pod. The design team that produced this radical design was headed by Clarence “Kelly” Johnson, a recent recruit at Lockheed, but already responsible for significant elements of the Hudson and Ventura, and later to work on the F-104, U-2 and SR-71.
After spending 1936 working on their design, Lockheed submitted it to the USAAC in April 1937. Two months later, on 23 June 1937, they were rewarded with a contract to produce a prototype. After eighteen months the prototype was complete. It was then moved from Lockheed’s factory at Burbank, California, to the nearby March Field air base, where the USAAC provided a test pilot, Lt. Benjamin Kelsey. He was an Air Corp project officer, heavily involved in pre-war fighter procurement.
The XP-38 was not a lucky aircraft. On its first taxi tests it suffered minor damage when inadequate brakes saw it skid off the runway. During its first flight, on 27 January, the flap mounts failed, and only Kelsey’s skill prevents a crash. Finally, on 11 February 1939 the aircraft was sent on a cross country flight. It reached Wright Field, Ohio safely. There Kelsey was met by General Hap Arnold, Chief of the Army Air Corps. Having averaged 360 mph on the cross-country journey, the XP-38 was well on track to set a new trans-continental speed record, and so Arnold sent Kelsey on to Mitchell Field, New York. Having reached there in record time, Kelsey was put in a queue to wait for a landing slot. While he was waiting, the XP-38’s carburettor appears to have iced up, forcing Kelsey to set down on a golf course, half a mile from the runway. While Kelsey survived intact, the XP-38 was written off, only sixteen days after its first flight, and with just under twelve hours in the air.
Over half of those hours had come on the trans-continental flight. On that flight, the XP-38 had achieved an average speed of 340 mph, and a top speed on the last section of 420 mph.
A number of changes were made to the design of the YP-38. The XP-38 had suffered from several problems, including engine overheating and tail buffeting. The engine overheating problems were easiest to resolve – the radiators on the sides of the engine booms were enlarged and new air scoops for the oil coolers were added below the engine nacelles.
The tail buffeting was more problematic. On the XP-38 the propellers had rotated in opposite directions, with the tips turning inboard at the top. This feature eliminated problems associated with engine torque in many aircraft. It was a particular problem on high powered fighters such as the Bf 109, which was notoriously difficult for novice pilots to land or take off as it tended to pull to one side. The counter rotating propellers of the P-38 allowed its pilots to apply power much more quickly on take off, giving it a very impressive initial rate of climb, and generally improving the stability of the aircraft. However, it was felt that the wash from the inwards rotating propellers was causing the tail buffeting, and so on the YP-38 the engines were swapped, so that the tips of the propellers turned towards the centre of the aircraft at the bottom. This had some impact on the problem, but did not solve it.
A second solution was to fit external mass balances on the elevators. This was in response to reports from army pilots, who described the problem as elevator flutter, and was opposed by Kelly Johnson, but these external balances would remain on all production aircraft. The real problem was later traced to problems with the air flow over the central section of the wing, between the two engines, and would be solved on the Lightning I.
The most visible change to the YP-38 came about as a result of a change of engines from the “C” Series engine used in the XP-38 to a more powerful “F” Series engine (Still the Allison V-1710). On the “C” series the propeller had been placed centrally on the engine, but on the “F” series it had to be placed nearer to the top of the engine. As a result the engine nacelles of the YP-38 and most subsequent models were taller and slightly less elegant that on the XP-38, with permanent oil cooler intakes under the nose of each engine replaced the retractable air scoops of the XP-38.
The YP-38 was to be armed with one 37mm cannon, two .50 inch machine guns and two .30 inch machine guns, all in the nose of the pilot pod. This unusual mix would eventually be replaced by one 20mm cannon and four .50 inch machine guns in the P-38E, the first truly combat ready version of the aircraft.
The first YP-38s were complete by September 1940. The first test flight of a YP-38 came on 17 September, and the aircraft were then delivered to Wright Field, where pilots from the 1st Pursuit Group would carry out a series of service tests. The resulting tests revealed a top speed of 405 mph at 20,000ft, slightly down from the more streamlines XP-38, but still very impressive. The first production aircraft, the P-38-LO, soon followed, deliveries beginning in June 1941.