Late in November 1940, Jack Northrop returned to the crew of three and twin tail/rudder assembly. To meet USAAC's request for more firepower, designers abandoned the ventral turret and mounted four 20 mm (.79 in) Hispano M2 cannon in the wings. As the design evolved, the cannon were subsequently repositioned in the belly of the aircraft. The P-61 therefore became one of the few U.S.-designed fighter aircraft to have a quartet of 20 mm (.79 in) cannon — along with the NA-91 version of the Mustang and the U.S. Navy's uprated F4U-1C Corsair as factory-standard in World War II. Northrop Specification 8A was formally submitted to Army Air Material Command at Wright Field, on 5 December 1940. Following a few small changes, Northrop's NS-8A fulfilled all USAAC requirements, and the Air Corps issued Northrop a Letter of Authority For Purchase on 17 December. A contract for two prototypes and two scale models to be used for wind tunnel testing (costs not to exceed $1,367,000), was awarded on 10 January 1941. Northrop Specification 8A became, by designation of the War Department, the XP-61. In March 1941, the Army/Navy Standardization Committee decided to standardize use of updraft carburetors across all U.S. military branches. The XP-61, designed with downdraft carburetors, faced an estimated minimum two-month redesign of the engine nacelle to bring the design into compliance. The committee later reversed the updraft carburetor standardization decision (the XP-61 program's predicament likely having little influence), preventing a potential setback in the XP-61's development. The Air Corps Mockup Board met at Northrop on 2 April 1941, to inspect the XP-61 mock-up. They recommended several changes following this review. Most prominently, the four 20 mm (.79 in) M2 cannon were relocated from the outer wings to the belly of the aircraft, clustered tightly with the forward-facing ventral "step" in the fuselage to accommodate them placed just behind the rear edge of the nose gear well. The closely spaced, centered installation, with two cannon stacked vertically, slightly outboard of the aircraft's centerline on each side, and the top cannon in each pair only a few inches farther outboard, eliminated the inherent drawbacks of the convergence of wing-mounted guns.