The concept of an economically-minded, very-lightweight-fighter was on the minds of warplanners across the globe even before the outbreak of World War 2 in September of 1939. The United States Army Air Corps (USAAC) was one branch of service looking into the prospect of a fleet of such aircraft designed as "point defense" solutions to counter the threat of a fleet of enemy bombers. The fighters would be cheap-to-produce en mass and quick-to-construct in the numbers needed. Compact dimensions, non-essential construction materials (namely wood), and suitable armament were at the heart of many of the contributions to this category of military aircraft.
As a private venture, the Douglas Aircraft Company took up a lightweight fighter design for the purpose of selling it to the Army Air Corps. Work began in 1939 and involved a basic, conventionally-arranged fuselage seating a sole pilot at midships. The engine was fitted to the nose in the usual way and a single-finned tail unit was at play at the rear. The mainplane consisted of a straight monoplane arrangement, though very slim in their design, and seated ahead of midships. The undercarriage was of a rather forward-thinking configuration, being tricycle (three-legged) and retractable. The main legs were installed at the fuselage which was rather unique for a fighter of the period where main legs were typically attached to the main wing members.
The engine of choice became the Ranger XV-770 V12 inverted liquid-cooled inline piston engine of 525 horsepower. This was installed in a compartment at the nose and drove a three-bladed 9.5 foot diameter blade. The aircraft carried a fuel capacity of just 50 gallons as range was not to be a concern for this point defense fighter system.
The aircraft, known internally as the Model 312, was purposely designed as compact, measuring a length of 21.8 feet with a wingspan of 32 feet and a height of 9 feet. Empty weight was 2,675lb against an MTOW of 3,400lb.
In terms of armament, engineers proposed a then-typical mix of a 0.30 caliber Browning medium machine gun with a 0.50 caliber Browning heavy machine gun - both weapons being air-cooled and belt-fed. These would be installed over the nose (and engine) and synchronized to fire through the spinning propeller blades.
On the whole, the fuselage carried all of the pertinent systems - pilot, engine, avionics, fuel, undercarriage and armament. The raised fuselage spine reduced vision to the rear but the aircraft would be flying most always with the enemy ahead of it. The interesting tricycle undercarriage may have added an engineering challenge and required a steady hand at the stick for the pilot when ground-running or landing / taking off.
Army authorities found enough value in the proposed Model 312 that a formal specification was drawn up around it in August 1939 to cover the expected "XP-48" prototype. However, this Douglas design survived for only a short time as the Army cancelled the project as soon as February of 1940 - apparently unconvinced at the promised 350 miles-per-hour top speed estimated by Douglas engineers.
Another lightweight point defense fighter project, the Tucker XP-57 (detailed elsewhere on this site) - also fell to naught and the Bell XP-77 (also detailed elsewhere on this site), another entry into this field, managed at least two prototypes which saw some time in the air before it, too, was cancelled (in 1944).