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The Douglas XA-42/ XB-42 Mixmaster was a twin-engined pusher aircraft that was one of the most advanced piston engined aircraft of the Second World War, but that was quickly superseded by jet powered aircraft.
The Mixmaster began as a private venture inspired by E. F. Burton, a designer at Douglas. He believed that it was possible to produce a twin engined aircraft that could carry 2,000lb of bombs to targets 2,000 miles away, with a top speed of 400mph. During the first half of 1943 Burton's team came up with a design for an aircraft with the two engines mounted within the fuselage, allowing them to have a clean wing and reduce drag. They also calculated that the new aircraft would be cheaper and easier to maintain than an equivalent number of B-29s.
While Burton and his team were working on this idea at Douglas, it was being proved in combat by the de Havilland Mosquito - on 30 January 1943 two Mosquito squadrons even managed to reach Berlin, disrupting speeches by Goering and Goebbels. The Mosquito went on to have an impressive record as a night bomber, with one of the lowest loss to mission ratios of any RAF aircraft.
Douglas submitted their new design in May 1943. The Bombardment Branch of Air Technical Service Command, USAAF, was impressed with the idea, and on 24 June 1943 Douglas was issued a contract to produce two prototypes and a static test airframe. At this point the aircraft was given the designation XA-42, putting it in the 'attack' category. This didn’t last for long, and on 26 November 1943 the designation was changed to XB-42 (B-42 was unallocated at the time, so the chance was taken to avoid confusion by using the same number).
The mock up of the XA-42 was completed in September 1943 and the first prototype was ready in May 1944, only ten months after the initial contract had been awarded. However this would still prove to be too late - by the middle of 1944 the B-29 was in production and US strategy in the Pacific was at least partly based around that aircraft and its requirements. It was unlikely that the XB-42 would enter production in time to make a major contribution to the war.
The XB-42 was a radical design for the time. It was powered by two 1,325hp Allison V-1710-125 engines, which were carried at a steep angle in the fuselage behind the pilot's compartment. A gear reduction box was carried in the tail cone, linked to the engines by five lengths of B-39 power shaft. Each engine drove its own three blade propeller, with the two rotating in opposite directions. The left engine drove the forward prop, the right engine drove the rear prop. This produced a churning effect behind the aircraft, which gave it the 'Mixmaster' name. Cooling was provided by air that entered via wing ducts.
The tail was mounted in front of the propellers, and was almost cross-shaped, with two vertical and two horizontal control surfaces. The lower vertical surface, below the fuselage, was about half the size of the upper vertical surface. This lower tail was installed to protect the propellers from damage in a belly landing.
The aircraft used tricycle landing gear, with the nose wing retracting under the cockpit. The aircraft carried a crew of three - the bombardier in the glazed nose, and the pilot and co-pilot side-by side under individual bubble type canopies (as used on the rather more massive Douglas XB-19). In many ways the resulting aircraft resembled the first generation of jet aircraft.
The XB-42 could carry double the payload of the Mosquito - four 2,000lb bombs could fit inside the internal bomb bay, or one 10,000lb bomb could be carried if the doors were left 5in open. It was also armed with remotely controlled 0.50in machine guns, carried in the rear of the wings. These guns were normally hidden behind doors to reduce drag. When in use they could cover quite a sizeable cone behind the aircraft.
The XB-42 made its maiden flight on 6 May 1944, and performed very well. Speed was as predicted, and the range and rate of climb were better than expected. There were some problems - the contra rotating propellers caused excessive vibration, worse when the bomb doors were open. Control forces were poorly harmonized, so more effort was needed to move on some directions than in others and the cooling wasn't adequate. Finally the twin bubble canopy made it difficult for the pilot and co-pilot to communicate. None of these problems were major, and Douglas worked on solving them as the flight test programme continued.
The XB-42 was an impressive aircraft, with a top speed of over 450mph, good payload and good range, but time was against it. It was already clear that the new turbojet engines would produce even more impressive designs, and so the USAAF decided not to put the B-42 into production.
A second prototype was completed during 1944, and made its maiden flight on 1 August 1944. It used a slightly newer version of the Allison engine and was soon given a single canopy. This aircraft set a trans-continental speed record of 433mph on a trip from Long Beach, California, to Bolling Field, DC, which took 5h4 17m 34sec. A few days after arriving in Washington the aircraft crashed, although the crew escaped safely.
The surviving first prototype was used to test turbojets. As far back as December 1943 Douglas had suggesting fitting two jet engines under the wings, and this was tried out after the end of the Second World War. The aircraft was redesignated as the XB-42A and given two 1,600lb/ 726kg thrust Westinghouse 19XB-21 axial-flow turbojets alongside two 1,375 Allison V-1710-133 liquid cooled engines. The aircraft made its first flight in this configuration at Muroc Dry Lake, California, on 27 May 1947, and reached a top speed of 488mph and a cruising speed of 442mph.
The XB-42A was used for tests, before being struck off charge on 30 June 1949. It was then preserved at the Smithsonian National Air & Space Museum, Washington.
The obvious next step was to replace the internal piston engines with turbojets. This produced the Douglas XB-43, the first US jet bomber
Engine: Allison V-1710-125 engines
Power: 1,325hp each
Crew: 3
Span: 70ft 6in
Length: 53ft 7in
Height: 18ft 10in
Empty weight: 20,888lb
Gross weight: 33,200lb
Maximum take-off weight:
Max speed: 410mph at 23,440ft
Service ceiling: 29,400ft
Normal Range: 1,800 miles
Armament: Four 0.50in machine guns,
Bomb load: One 10,000lb bomb or four 8,000lb bombs