Douglas XB - 42 Mixmaster The Douglas XB - 42 Mixmaster was an experimental bomber aircraft , designed for a high top speed . The unconventional approach was to mount the two engines within the fuselage driving a pair of contra - rotating propellers mounted at the tail , leaving the wing and fuselage clean and free of drag - inducing protrusions . Two prototype aircraft were built , but the end of World War II changed priorities and the advent of the jet engine gave an alternative way toward achieving high speed . Design and development . The XB - 42 was developed initially as a private venture ; an unsolicited proposal was presented to the United States Army Air Forces in May 1943 . This resulted in an Air Force contract for two prototypes and one static test airframe , the USAAF seeing an intriguing possibility of finding a bomber capable of the Boeing B - 29 Superfortress ' range without its size or cost . The aircraft mounted a pair of Allison V - 1710 - 125 liquid - cooled V - 12 engines behind the crew ' s cabin , each driving one of the twin propellers . Air intakes were in the wing leading edge . The undercarriage was tricycle and a full , four surface cruciform tail was fitted , whose ventral fin / rudder unit prevented the coaxial propellers from striking the ground . The pilot and co - pilot sat under twin bubble canopies , and the bombardier sat in the extreme front behind a plexiglass nose . Defensive armament was two 0 . 50 in ( 12 . 7 mm ) machine guns each side in the trailing edge of the wing , which retracted into the wing when not in use . These guns were aimed by the copilot through a sighting station at the rear of his cockpit . The guns had a limited field of fire and could only cover the rear , but with the aircraft ' s high speed it was thought unlikely that intercepting fighters would be attacking from any other angle . Two more guns were fitted to fire directly forward . Initially ordered as attack aircraft ( XA - 42 ) in the summer of 1943 , this variant would have been armed with 16 machine guns or a 75 mm ( 2 . 95 in ) cannon and two machine guns . Operational history . The first XB - 42 was delivered to the Army Air Force and flew at Palm Springs , California on 6 May 1944 . Performance was excellent , being basically as described in the original proposal ; as fast or faster than the de Havilland Mosquito but with defensive armament and twice the bombload . The twin bubble canopies proved a bad idea as communications were adversely affected and a single bubble canopy was substituted after the first flight . Testing revealed the XB - 42 suffered from some instability as excessive yaw was encountered , vibrations , and poor engine cooling - all problems that could probably have been dealt with . Due to the vertical stabilizer and rudder located underneath the fuselage , careful handling during taxiing , takeoff and landing was required because of limited ground clearance . The end of World War II , though , allowed the Air Force to consider possibilities with a little more leisure and it was decided to wait for the development of better jet bombers rather than continue with the B - 42 program . In December 1945 , Captain Glen Edwards and Lt . Col . Henry E . Warden set a new transcontinental speed record when they flew the XB - 42 from Long Beach , California to Bolling Air Force Base in Washington , D . C . ( c . 2 , 300 miles ) and in just 5 hours , 17 minutes , the XB - 42 set a speed record of 433 . 6 mph ( 697 . 8 km / h ) . The record - breaking XB - 42 prototype had been destroyed in a crash at Bolling Field attributed to a failure of the landing gear , but the other was used in flight test programs , including fulfilling a December 1943 proposal by Douglas to fit uprated engines and underwing Westinghouse 19XB - 2A axial - flow turbojets of 1 , 600 lbf ( 7 . 1 kN ) thrust each , making it the XB - 42A . In this configuration , it first flew at Muroc ( now Edwards Air Force Base ) on 27 May 1947 . In testing , it reached 488 mph ( 785 km / h ) . After 22 flights , the lower vertical stabilizer and rudder were damaged in a hard landing in 1947 . The XB - 42A was repaired but never flew again , and was taken off the AAF inventory on 30 June 1949 .