During May of 1943, Japanese authorities delivered a new requirement for a single-seat, single-engine long-range escort fighter to protect bomber formations from interception by Allied warplanes beginning to gain the advantage in the skies over the Pacific and Southeast Asia. The Mitsubishi Ki-73 by Tomio Kubo was one result of this requirement but the type was not furthered beyond a sole, incomplete prototype before the war's end in 1945. Kubo managed to find success with his earlier twin-engine Ki-46 "Dinah" and eventually moved on to the promising "Ki-83" twin-engine, long-range heavy fighter design of which four prototypes ultimately emerged when the Ki-73 was abandoned for good.
Rather unique for a Japanese-originated wartime fighter design was the use of a contra-rotating propeller arrangement in which two three-bladed systems were in play. These were driven by a single Mitsubishi Ha-203-II 24-cylinder, liquid-cooled "horizontal H" inline piston engine of 2,600 horsepower which was essentially two 12-cylinder engines paired and driving the twin propeller units. The rest of the overall design arrangement was conventional - the engine in the nose, a single-finned tail at rear and the cockpit set over center mass. Wings were straight monoplane appendages with clipped tips and the undercarriage was a "tail-dragger" system made fully retractable.
Issues with the early prototype form arose centering on the complex engine fit and there were growing concerns about the structure and control scheme particularly at the speeds anticipated. Eventually work on the troublesome compound engine was ended which, in turn, ended development of the Ki-73. While the Ki-73 was never formally adopted for service and never entered serial production, captured documents by the Allies - who believed the type was to come online soon - ushered in the codename of "Steve" for the series which never was.
The proposed maximum speed for the Ki-73 was 750 kilometers per hour, about 466 miles per hour, making it one fast mount aided through its powerful engine setup and contra-rotating blades. The slender, streamlined fuselage was also clean from nose to tail. Armament would have most likely been all-cannon in keeping with Japanese fighter traditions late in the war, perhaps 4 x 20mm systems buried in the wings (two units per wing).