The Long P-40 Line
The P-40 offered little new in design over the P-36 except a streamlined nose with an inline engine; in fact, the prototype was a P-36A with an Allison V-1710-19. Since the P-37’s high-altitude turbosupercharger was still unreliable, designer Don Berlin advocated a less complex medium-
altitude, single-stage blower built into the engine. He later complained that the P-37’s “supercharger wasn’t working, and we didn’t have time to develop that, too.”
Instead, on March 3, 1938, Berlin proposed to fit a P-36 with a 1,000-hp Allison using a single-stage geared supercharger, raising medium altitude speed without the turbo’s disadvantages. The Air Corps quickly responded on April 26 with a contract to modify the tenth P-36A to the XP-40. First flown October 14, 1938, by Ed Elliot, the XP-40 was further modified in February 1939 by moving the radiator from behind the wing to under the nose and installing the usual two guns in blast tubes over the engine.
The Army’s pursuit competition had called for bids on January 25, 1939, for designs whose top speeds of 310 to 370 mph were to be reached at 15,000 feet, with a two-hour endurance at cruising speed. Besides the XP-40, Curtiss also offered the P-36B, the P-37, and the Hawk 75R, while Seversky had the AP-4 and AP-9A with radial engines. The Seversky XP-41 arrived too late to compete.
Higher critical altitudes and speeds were offered by the turbo-superchargers on the Lockheed XP-38 and Bell XP-39 “interceptor-pursuits” designed for “cannon” armament, but neither had been tested. At that time, the Air Corps still preferred to mass-produce a medium-altitude pursuit first, and compared to the air-cooled types, the XP-40 was faster, less expensive, and available for rapid delivery.
According to the evaluation, of 1,000 possible points, Figure of Merit scores were: XP-40, 744 points; AP-4A, 713 points; H75R, 660 points; P-37, 622 points; P-36B, 604 points; and AP-9, 502 points.
Although Curtiss was the low bidder, and had guaranteed a 360-mph top speed at 15,000 feet, only 342 mph at 12,200 feet had been attained in tests.
On March 28, the Army ordered the XP-40 sent to the National Advisory Committee for Aeronautics (NACA) for tests in that government agency’s full-scale wind tunnel, where several ways of cleaning up the design’s streamlining were tried. Then the XP-40 was returned to the company on April 11 for refinements such as a new radiator arrangement under the nose, individual exhaust stacks replacing the one-piece manifold, a carburetor intake between and ahead of the gun muzzles, and improved landing gear. These features, plus flush riveting, were adopted on production P-40s.
The P-40 had arrived at the right time, for President Roosevelt was asking Congress to expand the Air Corps from 2,300 to 5,500 planes. He understood the influence of German air power on the Munich crisis, and that American air power needed to be rapidly increased. Only the P-40 was nearly ready for production, while the best of the others would need service tests.
The President signed the Air Corps Expansion Act on April 26, 1939, and on the same day, 524 Curtiss P-40 low-altitude pursuits were ordered for $12,872,398; the largest Air Corps contract to that date. Thirteen each of the YP-38 and YP-39 interceptor-pursuits were approved the next day. All would have Allison engines, which became the only American-designed liquid-cooled engine available in World War II, as none of the liquid-cooled projects begun by four other companies would ever reach mass production.