XP-39E with laminar flow wing
Between 1941 and 1944, there was no significant improvement in the P-39 aircraft. However, the Bell design team recognized the problem and focused on improving the aircraft’s performance, firepower, and maintenance in three directions. In February 1941, Bell representatives presented two improved variants of the P-39, the XP-39E, and XP-63, at Wright Field. The XP-39E specification was published in March, and it was set to receive a supercharged V-1710-E9 engine, a 37mm cannon, and six 0.50in machine-guns. A month later, the USAAF ordered two prototypes under contract No. AC18373 for $513,750.
The first XP-39E prototype (sin 41-19501) made its maiden flight on February 21, 1942, at Niagara Falls Airport. It was equipped with a V-1710-E4 engine as the E9 variant was not yet available. The XP-39E had several modifications compared to the standard P-39, including laminar flow aerofoil wings with squared tips and a 22in increase in span. The fuselage was 20in longer, and the tail unit had been completely redesigned. The air intake of the carburettor had been moved aft on the back of the fuselage, and the size of the airscoops at the wing roots had been increased. The undercarriage had been strengthened to accommodate the aircraft’s increased weight, which was now 6,951 lbs compared to 5,474 lbs for the P-39D. However, the XP-39E’s weight increase was not compensated for by the more powerful engine, as spinning tests conducted after 35 flights and 14hr 55min resulted in the aircraft’s total destruction on March 26, 1942.
The second prototype (41-19502) was equipped with a V-1710-E9 and took its first flight on April 4, 1942. Unfortunately, it crash-landed near Spencerport, New York, during its 27th flight on May 15, 1942. The nose undercarriage and propeller were seriously damaged, and tests had to be stopped as there were no other prototypes available. Consequently, a third prototype (42-71464) was ordered under a $271,735 contract on May 27, 1942. The third aircraft completed its maiden flight on September 19, 1942, and logged 5hr 40min flying time before being transferred to Allison for engine tests. The second prototype was repaired but was involved in another accident on February 8, 1943.
Test flights showed that the XP-39E did not have any significant improvements in performance when compared to the P-39D. The XP-39E had a slower climb, a lower service ceiling, a higher landing speed, and a longer take-off distance. Although its maximum speed was higher, it accelerated slower. As a result, the XP-39E program was terminated, and the contract orders for 4,000 production aircraft, to be designated P-76, were cancelled. Both aircraft were later used in the development of the XP-63.