The nickname "Lightning" was given to the P-38 by the RAF when the aircraft was ordered by them in 1940. The Americans adopted the same name the following year.

The first P-38 production runs sold to the British did not include a supercharger fitted to their Allison V-1710 engines. The supercharger (turbocharger) was deemed to be classified equipment by the U.S.  Government, and was thus restricted from overseas sales. These P-38's performed poorly, resulting in the order for 667 additional P-38s being cancelled by the RAF. 

P-38 models E, F, H, J, and L were fitted with turbocharged engines, improved Fowler flaps and additional fuel capacity. These P-38s proved to be more than a match for Axis fighters across the globe.

This fighter filled many diverse roles. In addition to its primary role as a fighter aircraft, it was used for photo reconnaissance (F-4 & F-5), as a night fighter with added radar and a second crew member, an ambulance, a torpedo-bomber, and as a target tug. It made its first flight on January 27, 1939 from designs dating back to 1937. There were 9,924 of all versions of this aircraft produced.

• First Flight Date: 27 January 1939
• Type: Single-seat fighter and fighter-bomber
• Powerplant(s): Two turbocharged Allison 1,427 hp V-1710-89/91 (P-38J) or two 1600 hp V-1710-111/113 (P-38L/M) in-line piston engines.
• Max Speed: 414 mph at 25,000 ft.
• Range: 475 miles with internal fuel tanks, 2600 miles with improved fuel capacity tanks.
• Service ceiling: 44,000 ft.
• Weapons: One 20 mm cannon and four 0.50 cal. machine guns, plus up to 3,190 lbs of ordnance - usually two 1,000 lb or 1,600 lb bombs or 10x127 mm high-explosive rocket projectiles under the wings.
• Weights: Empty 12,780 lbs; loaded (Max Take-off Weight) 21,600 lbs.
• Dimensions:
  
  • Wing Span: 52 ft 0 inches.
  • Wing Area: Approx. 327 sq. ft.
  • Length: 37 ft 10 inches.
  • Height: 9 ft 10 inches.

The P-38 was the first American Army Air Corps fighter able to exceed speeds of more than 400 mph. The twin turbocharged engines gave it superb high-altitude performance, and at altitudes above 26,000 feet was one of the world's best fighters.

The concentration of weaponry in the P-38's nose allowed it to bring immense firepower to bear on targets at all ranges. Lightnings could saw Japanese Zeros in half, or completely severe an enemy aircraft's wing with a long burst of fire.

Ref: 1) Jane's Historic Military Aircraft   2) Aircraft of the World