In early 1939, the Douglas Aircraft Company of Santa Monica, California submitted a proposal to the USAAC for an ultra-lightweight single-seat high-altitude fighter. The project was given the company designation of Model 312. The general arrangement drawings of the Model 312 that have survived show a rather unusual-looking low-wing cantilever monoplane sporting a wing with a rather high aspect ratio. Power was to be provided by a supercharged Ranger SGV-770 twelve-cylinder inverted-vee liquid-cooled engine offering 525 hp. A three-bladed propeller was to be used. A tricycle undercarriage was to be fitted. The wing was so thin that the main undercarriage members had to be attached to the fuselage, the mainwheel members retracting rearward into recesses within the rear fuselage. Armament was to consist of a 0.30-in and a 0.50-in machine gun, both mounted in the upper fuselage deck and synchronized to fire through the propeller arc.
The dimensions of the proposed Model 312 were to be wingspan 32 feet, length 21 feet 9 inches, height 9 feet, and wing area of 92 square feet. Weights were only 2675 pounds empty and 3400 pounds gross. The Douglas designers projected a maximum speed of no less than 525 mph for the Model 312 design!
The USAAC looked over the Douglas proposal, and were sufficiently interested that they reserved the pursuit designation of XP-48 for the design. However, upon further investigation, the USAAC concluded that Douglas' performance estimates were grossly over-optimistic, and the project was not funded. Consequently, the Douglas company pursued the Model 312 project no further.