The Douglas Invader has a unique history. It was one of very few aircraft to be entirely conceived, designed, developed, produced in quantity and used in large numbers, all during World War II. The whole programme was terminated after VJ-Day and anyone might have judged the aircraft finished. With new jets under development, Douglas made no effort to retain any design team on Invader development, neither did the Army Air Force show any interest. Yet this aircraft proved to be of vital importance in the Korean War and again in Vietnam and, by 1963, was urgently being remanufactured for arduous front-line service. Many remain in combat units 30 years after they were first delivered, a record no other kind of aircraft can equal.
The design was prepared by Ed Heinemann at El Segundo as a natural successor to the DB-7 family, using the powerful new R-2800 engine. The Army Air Corps ordered three prototypes in May 1941, one with 75 mm gun, one with four 20 mm forward-firing cannon and four 0.5 in guns in an upper turret, with radar nose, and the third as an attack bomber with optical sighting station in the nose and two defensive turrets. In the event it was the bomber that was bought first, designated A-26B. Much faster than other tactical bombers with the exception of the Mosquito, it was 700 lb lighter than estimate, and capable of carrying twice the specified bomb load. It was the first bomber to use a NACA laminar-flow airfoil, double-slotted flaps and remote-control turrets (also a feature of the B-29).
Combat missions with the 9th AF began on 19 November 1944 and these aircraft dropped over 18,000 tons of bombs on European targets. A total of 1,355 A-26Bs were delivered, the last 535 having R-2800-79 engines boosted by water injection. The A-26C, in service in January 1945, had a transparent nose, lead-ship navigational equipment and was often fitted with H2S panoramic radar. In 1948 the B-26 Marauder was retired from service and the Invaders were redesignated B-26. Over 450 were used in Korea, and in Vietnam these fine aircraft were one of the most favoured platforms for night attack on the Ho Chi MInh trail and in other interdiction areas. Though top speed was depressed to about 350 mph, the A-26A (as the rebuilt B-26K was called) could carry up to 11,000 lb (4,990 kg) of armament, deliver it accurately and, with 2 hours over target, over a wide radius. In 1976 eight air forces around the world still retained Invader squadrons, a fitting tribute to a truly remarkable aircraft.