US Army Air Forces Circular X-608 was drawn up in February 1937 as a proposal for a twin-engine interceptor designed to attack enemy bombers at high altitude.
The framers of the specification (Lieutenants Benjamin S. Kelsey and Gordon P. Saville) drew it up in such a way to get around the Army's strict definition of a "pursuit" aircraft as a single-seat plane with only one engine, carrying no more that 500 pounds of total armament and ammunition. By giving the role of the new plane as an interceptor, they were able to use two engines (of a greater horsepower) and carry up to 1,000 pounds of armament and ammunition. The other specifications were very tough:
A top speed of 290 mph at sea level and 360 mph at 20,000 feet
A climb to 20,000 feet in 6 minutes or less
An endurance at 20,000 feet of one hour at full throttle
The ability to land and take off over a 50-foot obstacle within 2,200 feet
A requirement to use Allison V-1710 engines of 1,000 horsepower with General Electric B-1 superchargers
A similar Circular Proposal X-609 issued around the same time (under most of the same specifications; Allison engine, 360 mph top speed at 20,000 feet, etc.) for a single-engine interceptor resulted in the Bell P-39 Airacobra.
The competing designs would also receive additional consideration if they used a tricycle landing gear. A large internal fuel capacity was also a requirement; the Army Air Forces would not accept the need to use any kind of external fuel tank, as their use was banned by edict in May 1939. Boeing, Consolidated, Curtiss, Douglas, Lockheed and Vultee were all queried about submitting designs. Clarence "Kelly" Johnson and Hall L. Hibbard of Lockheed came up with a variety of designs to house the two engines and their superchargers while attempting to incorporate the tricycle landing gear. Lockheed promised a top speed of over 400 mph, as the new plane was designed to accept engines of up to 1,150 horsepower. The Army Air Forces were skeptical of Lockheed's claim, but the Lockheed design (Design number "4" in the above image) was selected, becoming the experimental XP-38 on June 23, 1937. It first flew on January 27, 1939. Lockheed delivered spectacularly; the XP-38 had a top speed of 413 mph at 20,000 feet, and could climb to that height in under six minutes. The XP-38 had an empty weight of 11,507 lb., gross weight of 13,964 lb., and maximum takeoff weight of 15,416 lb. The XP-38, as it was just a prototype, was not fitted with any armament. The armament requirement as specified by Saville and Kelsey for the submitted X-608 proposals was one Hotchkiss 25 mm aircraft cannon and four .50 caliber Browning machine guns. The supposed Hotchkiss cannon turned out to not actually exist, and so the XP-38's nose bay was re-designed to accommodate a T1 23 mm cannon with a rotary magazine instead. The central-nacelle design of the XP-38 allowed the armament to be clustered in the nose, allowing for much greater accuracy and range. The XP-38 crashed while attempting to land, and was scrapped. Impressed with the XP-38 in spite of the crash, in 1939 the Army ordered thirteen more YP-38 prototypes for testing. The YP-38 first flew on September 16, 1940. Most of the the thirteen YP-38s were fitted with armament; what was eventually agreed upon was two .50 caliber Browning machine guns with 200 rounds per gun, two .30 caliber Browning machine guns with 500 rounds per gun, and a hefty T9 37 mm cannon with 15 rounds. The P-39 Airacobra also used this gun. Even with the additional weight of the armament being installed, the YP-38 was lighter as a result of redesigns; the empty weight was 11,171 lb., the gross weight was 13,500 lb., and the maximum takeoff weight was 14,348 lb. Even before the YP-38s had finished production, the Army ordered 66 more and standardized the aircraft as the P-38. In the P-38, the machine gun portion of the armament was changed back to four Browning .50 caliber machine guns. One was chosen to be completed with an experimental pressurized cockpit and designated the XP-38A; it presumably would have become the P-38A had the Army chose to produce it.
The designations P-38B and P-38C were not used. After the first 30 P-38 aircraft were built, the P-38D appeared, and was fitted with additional equipment to make it more "combat ready". The remaining 36 aircraft of the order were built as such. The P-38E, along with other improvements, replaced the T9 37 mm cannon of the P-38 and P-38D with a Hispano 20 mm cannon, changing the basic armament of the fighter for the final time.
The prototype YP-38s initially had issues when diving (the P-38 dived at a speed that few, if any, aircraft had achieved before) caused by the tail shaking severely and even breaking off, along with the controls locking up and the nose forcefully moving down; a YP-38 crashed, killing a test pilot. At first, assuming tail flutter, a set of mass balances were fitted to the horizontal tail, but these did not help at all. Nevertheless, they continued to be fitted to all P-38s. It was later found that the tail "flutter" was related to the problem of compressibility, which plagued the early P-38 in combat over Germany in late 1943. Compressibility flaps, which modified the airflow over the P-38's wing and allowed the pilot to keep control, were fitted to the underside of the wings outboard of the engines from the P-38J-25-LO block onwards. The J-25-LO also introduced power-boosted ailerons. With them, the pilot only had to use 17 percent of the stick force when compared to a P-38 that did not have them. This allowed the P-38 to have the highest roll rate of any fighter during WWII.
| Variant | Number built | Serial numbers |
|---|---|---|
| XP-38 | 1 | 37-457 |
| YP-38 | 13 | 39-689/701 |
| P-38-LO | 29 | 40-744/761, 40-763/773 |
| XP-38A | 1 | 40-762 from above contract modified as XP-38A |
| P-38B-LO | 0 | -- |
| P-38C-LO | 0 | -- |
| P-38D-LO | 36 | 40-774/809 |
| P-38E-LO | 210 | 41-1983/2097, 41-2100/2120, 41-2172, 41-2219, 41-2221/2292 |
Sources:
Joe Baugher on the P-38, referencing;
Lockheed Aircraft Since 1913, Rene J. Francillon, Naval Institute Press, 1987
The P-38J-M Lockheed Lightning, Profile Publications, Le Roy Weber Profile Publications, Ltd, 1965.
War Planes of the Second World War, Fighters, Volume Four, William Green, Doubleday, 1964.
Famous Fighters of the Second World War, William Green, Doubleday, 1967.
The American Fighter, Enzo Anguluci and Peter Bowers, Orion Books, 1987.
Wings of the Weird and Wonderful, Captain Eric Brown, Airlife, 1985.
United States Military Aircraft since 1909, Gordon Swanborough and Peter M. Bowers, Smithsonian Institution Press, 1989.
Der Gabelschwanz Teufel: Assessing the Lockheed P-38 Lightning, by Dr. Carlo Kopp, P.E.