The XP-67 was McDonnell's only propeller-powered fighter. On Aug. 14, 1941, the U.S. Army Air Forces placed its first order for two prototypes of a novel, twin-engine, long-range fighter with a pressurized cabin. Engineers tried to improve the airplane's aerodynamics by merging the center fuselage with rear portions of the engine nacelles. This resulted in the XP-67's unique bat-like shape. The turbo-supercharged 12-cylinder, inverted-V, liquid-cooled engines would be housed in long nacelles and would drive four-blade propellers in opposite directions. They were to maximize the use of exhaust to increase engine thrust. The XP-67 was completed in St. Louis, Mo., in December 1943, but the temperamental engines caused the airplane's first flight to last only six minutes. Problems continued with the engines, and the XP-67's top speed was 200 mph slower than required. The second prototype was never finished.