maintain airspeed; if a pilot did that in the P-38, regardless of which engine had failed, the resulting engine torque and p-factor force produced a sudden, uncontrollable yawing roll, and the aircraft would flip over and hit the ground. Eventually, procedures were taught to allow a pilot to deal with the situation by reducing power on the running engine, feathering the prop on the failed engine, and then increasing power gradually until the aircraft was in stable flight. Single-engined takeoffs were possible, though not with a full fuel and ammunition load. The engines were unusually quiet because the exhausts were muffled by the General Electric turbosuperchargers on the twin Allison V12s. Early problems with cockpit temperature regulation occurred; pilots were often too hot in the tropical sun as the canopy could not be fully opened without severe buffeting, and were often too cold in Northern Europe and at high altitude, as the distance of the engines from the cockpit prevented easy heat transfer. Later variants received modifications (such as electrically heated flight suits) to solve these problems. On 20 September 1939, before the YP-38s had been built and flight tested, the USAAC ordered 66 initial-production P-38 Lightnings, 30 of which were delivered to the (renamed) USAAF in mid-1941, but not all these aircraft were armed. The unarmed aircraft were subsequently fitted with four .50 in (12.7 mm) machine guns (instead of the two .50 in/12.7 mm and two .30 in/7.62 mm of their predecessors) and a 37 mm (1.46 in) cannon. They also had armored glass, cockpit armor, and fluorescent instrument lighting . One was completed with a pressurized cabin on an experimental basis and designated XP-38A. Due to reports the USAAF was receiving from Europe, the remaining 36 in the batch were upgraded with small improvements such as self-sealing fuel tanks and enhanced armor protection to make them combat-capable. The USAAF specified that these 36 aircraft were to be designated P-38D. As a result, no P-38Bs or P-38Cs were designated. The P-38D's main role was to work out bugs and give the USAAF experience with handling the type. In March 1940, the French and British, through the Anglo-French Purchasing Committee, ordered a total of 667 P-38s for US$100M, designated Model 322F for the French and Model 322B for the British. The aircraft was a variant of the P-38E. The overseas Allies wished for complete commonality of Allison engines with