The Douglas A-26 Invader was the best medium bomber to see service with the USAAF during the Second World War, but production delays meant that it wasn't available in large numbers until late in 1944, and it was only used in significant numbers by the Ninth Air Force in Europe. It then became a mainstay of the post-war Air Force, replacing the A-20, B-25 and B-26 in service, and making a major contribution to the air war over Korea before ending its military career in the skies over Vietnam.
Work on the A-26 seems to have been triggered by a letter sent to Douglas on 5 November 1940 by Major Frank O. Caroll, chief of the Air Corps' Experimental Engineering Section at Wright Field, possibly in the aftermath of a visit to the base by Edward H. Heinemann, a senior Douglas designer. The letter gave a list of those features of the Douglas A-20 that the Air Force thought needed to be improved in a new bomber. The A-20 was considered to have five main faults
After his trip to Wright Field Heinemann, who was the designer and patent holder for the A-20, began to work on a design for a bomber capable of carrying a 75mm cannon. At the same time R.G Smith, an engineer and artist, and Arthur Raymond, the engineering vice president of Douglas, began to work on the same project.
By the end of the war the A-26 had been accepted in smaller numbers than any of the other main eighteen combat aircraft used by the Army and Navy. By August 1945 a total of 2,446 A-26s had been accepted. Next came the 3,760 Boeing B-29 Superfortresses, followed by the 5,157 B-26 Marauders. In Douglas's defence the A-26 was the last of the twelve main Air Force combat aircraft to enter development, and it and the B-29 were the only two to be developed and enter production after Pearl Harbor.
Work on the A-26 began before Christmas 1940. It made its maiden flight in July 1942 but didn't enter production until September 1943 and another year would pass before enough aircraft were available for it to enter combat. Only by 1945 production was finally running well. The Army Air Force tended to blame Douglas for the delays, although some, if not all, were beyond their control, and the Army's inability to decide how it wanted the aircraft to be armed was also partly to blame.
Heinemann's basic design was ready early in 1941. It used a laminar-flow airfoil provided by NACA, a new aluminium alloy that reduced weight, and slotted wing flaps with multiple airfoil shaped panels that Heinemann claimed with 30% more efficient than the flaps of the A-20 and reduced landing speeds by 10%. Firepower was provided by two remote controlled turrets aimed by a gunner in an internal rear compartment. The turrets could thus be shorter than manned turrets, reducing drag. NACA inspected the design, and helped with wing tunnel tests, and there were very few differences between the mock-up inspected by the Army in April 1941 and the first prototypes.
On 2 June 1941 Douglas was given contract W535 ac-17946, for one XA-26 bomber and one XA-26A night fighter, at a price of just over two million dollars. Later in June a third prototype, the XA-26B armed with a 75mm cannon, was added to the order, and by the end of October 1941, well before the maiden flight of the prototype, Douglas received a contract for the first 500 A-26s.
Work on the A-26 was plagued by delays almost from the start. Douglas had hoped that the new aircraft would make its maiden flight on 15 January 1942, but by May the Air Force was noting that an inability to get suitable landing gear struts was going to delay that flight until 1 July. Douglas also had problems with the late delivery of self sealing fuel tanks and the remote controlled turrets. The Army Air Force tended to blame Douglas for the delays, but some problems were caused by late delivery of government-furnished equipment, which included such essential features as the engines, propellers, spinners and electrical generators.
The Army Air Force's inability to decide exactly how they wanted the A-26 to be armed also caused some delays. In the summer of 1942 they decided that the first 500 aircraft would carry the 75mm cannon, and also ordered 200 gun noses carrying six .50in machine guns, which could be installed in the field. A series of experiments were carried out with different combinations of guns, using 75mm, 37mm and 20mm cannon and .50in machine guns. On 17 March 1943, when a second contract was issued for 500 more aircraft, the 75mm cannon was still in favour, but eventually only the XA-26B prototype would carry the big gun, and the .50in machine gun nose became the standard for the A-26B.
The XA-26 finally made its maiden flight on 10 July 1942. The test pilot declared that it was already ready for combat, but Douglas didn't expect deliveries to begin until July 1943. Even this was too optimistic, and the aircraft didn't go into mass production until September 1943. The biggest delays were now being caused by a shortage of production tools, especially milling machines capable of creating the Invader's wing spars – with so many new aircraft factories being built and so many different aircraft having high priority, it was hardly surprising that some bottlenecks would remain, and the Invader's wing spars were of a very different design to any other. These delays meant that the A-26 was not available in large enough numbers to enter combat until the autumn of 1944, when it began to enter service with the Ninth Air Force in Europe.
The A-26 made its combat debut in July 1944 with the Fifth Air Force on New Guinea. Four early aircraft were tested out, and the feedback was overwhelming poor. The streamlined cockpit put the pilot between the engine nacelles, greatly limiting visibility. This meant that the A-26 was unsuitable for the low-level formation flying needed in the Pacific, and also made it difficult to spot well hidden Japanese positions in the jungle. General George Kenney, the commander of the Fifth Air Force, stated that 'We do not want the A-26 under any circumstances as a replacement for anything'
Douglas responded to this feedback by producing a new raised cockpit canopy, which improved visibility, and by the summer of 1945 Kenney was willing to take the A-26. In May 1945 the Army Air Force decided that seven A-26 groups would redeploy to the Pacific from Europe, while all existing medium and light bomb groups in the Pacific, with the exception of three B-25 units, would convert to the A-26. In the event the war in the Pacific ended before this ambitious program began, and only a handful of A-26s played an active part in the war against Japan.
The A-26 made a better impression with the Ninth Air Force in Europe. Eighteen aircraft flow on a series of missions in September 1944, and the feedback was positive. The Ninth Air Force used its aircraft on medium level raids, where the poor visibility was less of a problem. The A-26 was faster and had a longer range than the A-20 or B-26, and General Hoyt Vandenberg, the commander of the Ninth Air Force, was happy to replace his existing medium bombers with the Invader. The Ninth Air Force would become the most important Second World War user of the Invader.
In December 1944 the Air Force placed two new contracts, for 2,400 aircraft, soon followed by more contracts for 1,600 aircraft including 350 of the improved A-26D, bringing the total number of aircraft ordered up to 5,000. Less than half of these aircraft were ever built. Production was scaled down dramatically in May 1945, and stopped with the 1,355th A-26B and the 1,091th A-26C. A single A-26D was produced, probably by converting an existing aircraft, giving a total of 2,446 production aircraft and three prototypes.
The A-26 ended the Second World War with a positive reputation. Major General S. E. Anderson of the Ninth Bombardment Division reported that his pilots came to view it as the best aircraft they had ever flown, while even General Kenney came to appreciate the firepower offered by later aircraft with six guns in the wings and eight in the nose. The A-26 went on to serve in large numbers in Korea (as the B-26) and in smaller numbers in Vietnam (initially as the B-26 and later as the A-26A). Despite its troubled development the A-26 Invader remained in front line service for twenty five years, and was not withdrawn until November 1969.
Three prototypes were built. The XA-26 (serial number 41-19504) was first to be completed, and resembled the A-26C, with a Plexiglas bombardier's nose. It was armed with the remote control upper and lower turrets. It made its maiden flight on 10 July 1942, and was used for most of the early flight tests of the Invader.
The second prototype, the XA-26A (serial number 41-19505) was for the night-fighter version of the Invader. Douglas line drawing show it with a four gun mid-upper turret, but it was built with four 20mm cannons carried in a ventral tray below the bomb bay, a longer radar carrying nose, two crewmen and no turrets. The XA-26A was abandoned when tests showed that it was no better than the Northrop XP-61 Black Widow, which had been undergoing flight tests for two months by the time the XA-26 made its maiden flight.
The third prototype, the XA-26B (serial number 41-19588) was added to the program just after the first two, carried a crew of three and was armed with a solid nose that could carry a wide number of different guns, from .50in machine guns up to a massive 75mm cannon. The XA-26B was the only aircraft to carry the 75mm gun, on the right side of the nose and protected by a retractable cover. The same 75mm gun was installed on the B-25 Mitchell, and proved to be disappointing in service, partly because of its slow rate of fire and partly because suitable targets were often rare.
In 1947 the Air Force abolished the A for Attack category of aircraft, and gave all of the remaining attack aircraft new designations. In a move that has caused endless confusion ever since it was decided to reuse the B-26 designation, previous used by the B-26 Marauder. By 1947 the last Marauders had been retired, so this must have seemed like a much less confusing option than the alternative, which would have been to give the Invader the next available B designation – by 1947 the sequence had moved into the high 40s or low 50s, but as time has passed the existence of two quite similar twin-engined medium bombers that had been service at the same time causes some confusion – a book on the B-26 in the Second World War is discussing a different aircraft to a book on the B-26 in the Korean War!
An extra level of confusion was introduced during the Vietnam War, when the B-26K was redesignated as the A-26A, more than twenty years after that designation had been allocated to the un-built night fighter version of the Invader, a move designed to satisfy the Thai government, which was willing to host attack aircraft but not bombers.