The last and the first

Prior to and during World War II, Douglas Aviation produced superb twin engine bombers, notably the A-20 and A/B-26 twin engine bombers, the latter of which served both in the Korean War and well into the Vietnam War.

 But Douglas’ most ambitious project was the XA-42, a mid-World War II private venture for a “super Mosquito,” a twin-engined bomber with a maximum speed in excess of 400 mph, a bombload of 4,000 pounds and a phenomenal combat radius (not range) of 2,000 miles.

To provide the cleanest possible airframe, the XA-42 mounted two Allison 1,325 horsepower liquid-cooled V-12 engines in the fuselage driving dual contra-rotating pusher propellers in the extreme tail. The tail surfaces were a cross with a lower fin and rudder underneath the tail to prevent the propellers from striking the ground during takeoffs and landings. The sleek laminar-flow wing was mid-mounted and the tricycle landing gear  retracted into the fuselage to keep the wing clean.

Two versions were intended — a “strafe” version with 16 .50 machine guns in a solid nose and a bomber version with a navigator/bomb-aimer in a standard glazed nose section. The pilot and co-pilot/gunner sat under small, separate bubble canopies, giving the XA-42 a “bug eye” appearance. Its defensive armament was four .50 machine guns, two each in two low drag remotely controlled turrets mounted mid-wing. It was assumed that the bomber’s high speed would prevent any enemy fighter attacks except from the rear, and the co-pilot/gunner’s seat would turn rearward to control the turrets to counter such an attack.

Not surprisingly, the engine arrangement was complex. The engines were slanted inside the fuselage and each drove one of the three-blade contra-rotating propellers, the left powerplant driving the forward propeller and the right the aft.

To turn the propellers, the power was transmitted via five lengths of shafting connected to a complex gearbox arrangement. Cooling to the radiators was provided by narrow slots cut into the leading edges of the inner wings.

The proposal was quickly named “Mixmaster” for its propeller arrangement and was submitted to the U.S. Army Air Force in May 1943. On June 25, 1943, a contract was issued for two flying prototypes and one static test airframe.

At this point, the USAAF began to consider the Douglas proposal as a possible high speed bomber that could match the range and bomb load of the problem prone B-29 at only a fraction of the cost, and the designation was changed to XB-42.

Progress was quite rapid. The first prototype flew on May 6, 1944, and proved to have outstanding performance, faster than the Mosquito and carrying twice the bombload. But there were problems, especially with the complex engine arrangement. Cooling air was inadequate and the long propeller shafts produced heavy vibration, especially when the bomb-bay doors were open. Other, lesser problems were poorly harmonized controls, excessive yaw and the twin canopies interfered with pilot/co-pilot communication.

The second XB-42 prototype replaced the twin canopies with a single canopy and, in December 1945, flew nonstop from Long Beach, Calif., to Washington, D.C., at an average speed of 433.6 mph. But by then the war was over and the USAAF had decided to move to higher-performance jet-powered bombers, so the XB-42 was the last twin-engine propeller driven bomber tested by the Army Air Force.

However, the internal mounting of the XB-42’s engines seemed to make it ideal for turbojet propulsion, and Douglas sold the Air Force on the idea that modifying the XB-42 into a jet bomber would be a relatively straightforward process that would save time and money compared to a brand new design. USAAF amended the XB-42 contract in March 1944 to include the development of two turbojet-powered XB-43s, the “Jetmaster,” and a contract was signed in the spring of 1945.

Douglas replaced the two Allison V-1710 engines with a pair of two General Electric TG-180 4,000 pounds/thrust axial flow turbojets and cut two air intakes into each side of the fuselage. Thus, the XB-43 became America’s first all-jet bomber.

The centerline mounting of the two engines provided very little asymmetrical thrust and additional safety in case one engine failed — a common occurrence with the new jet engines. Since there were no propellers to protect, the bottom vertical fin was also removed and the upper vertical fin was enlarged.

As Douglas predicted, the structural work was not overly complex, but it took almost two years for the prototype to fly because of delays with the engines. When the engines were finally installed, during a ground test the compressor blades on one failed catastrophically, ripped through the engine casing and severely damaged the airframe, requiring a seven-month delay ensued for repairs.

The XB-43 finally flew on May 17, 1946, and had the distinction of being America’s first turbojet bomber, but tests showed unimpressive performance. The USAAF was already moving ahead with a new bomber, the North American XB-45 Tornado, designed from the outset for jet power and promising a quantum leap in every category of  performance, and the XB-43 program was canceled, though the prototypes did useful service for many years as an engine test bed.

(For questions or comments, contact Dr. Michel at marshall.michel@ramstein.af.mil.)