At the time when the Curtiss P-40 fighter was initially entering production, Curtiss's chief designer Donovan Berlin was already thinking about its successor. The P-40 was already largely obsolescent by contemporary European standards even before it had entered production, and early war experience in Europe suggested that more speed, more protection, and more firepower would very soon be required.
Influenced by contemporary British and French thinking, Berlin submitted his ideas to the USAAC. The USAAC was sufficiently impressed that they issued a Circular Proposal (CP 39-13) based on Berlin's proposal. The Army ordered two prototypes from Curtiss under CP 39-13 on September 29, 1939. The designation was XP-46 and the serials were 40-3053 and 40-3054.
The XP-46 was generally similar to its P-40 predecessor, but was somewhat smaller and featured a wide-track, inwardly-retracting undercarriage. The engine was to be the newly-developed Allison V-1710-39 (F3R) twelve-cylinder liquid-cooled Vee of 1150 hp. This same engine was also later to power the D-version of the P-40. In view of the relatively high wing loading, automatic leading-edge slots (a la Bf 109E) were fitted to the outer portions of the wing to give increased aileron control near the onset of the stall. Armament was to be two 0.50-in machine guns in the nose below the cylinder banks and no less than eight 0.30-inch machine guns in the wings. This made the XP-46 the most heavily-armed American fighter up to that time. A month after the initial XP-46 order, the USAAC modified their requirement and called for the provision of self-sealing fuel tanks and armor protection for the pilot. The maximum speed when fully armed and armored was to be a rather ambitious 410 mph at 15,000 feet.
In order to save time and get something in the air as quickly as possible, the second prototype (40-3054) was delivered without armament or radio. This aircraft was redesignated XP-46A. The XP-46A was actually the first to fly, taking to the air on February 15, 1941. Even with all the military equipment taken off, the XP-46A was just barely able to achieve 410 mph at 12,200 feet, the required maximum speed when fully equipped.
When the fully-equipped XP-46 flew for the first time on September 29, 1941, the additional weight of the military equipment slowed the fighter down to only 355 mph at 12,200 feet.
In the meantime, while the XP-46 and XP-46A prototypes were still under construction, the USAAC decided in June of 1940 not to order the P-46 into production, but rather to order a similarly-powered version of the already-existing P-40. This was eventually to emerge as the P-40D. This option had the advantage in not disrupting Curtiss production lines by the introduction of a completely new airframe at a critical period. In the event, this turned out to have been a wise decision, since the fully-equipped XP-46 was actually slower than the P-40D.
Specification of the Curtiss XP-46:
One 1150 hp Allison V-1710-29 liquid-cooled engine. Armed with eight 0.3-inch machine guns in the wings and two 0.50-inch guns in the nose. Maximum speed of 355 mph at 12,300 feet. Climb to 12,300 feet in 5 minutes. Service ceiling of 29,500 feet. Range at maximum cruising speed was 325 miles. Weights were 5625 pounds empty, 7322 pounds loaded, and 7665 pounds maximum. Dimensions were wingspan 34 feet 4 inches, 30 feet 2 inches long, 13 feet 0 inches high, and wing area 208 square feet.