twin-engined Luftwaffe combat aircraft. The first drop tanks were designed to be discarded when empty or in the event of combat or emergency to reduce drag, weight, and to increase maneuverability. Modern external tanks may be retained in combat, to be dropped in an emergency. The Allies commonly used them to allow fighters increased range and patrol time over continental Europe. The RAF used such external fuel tanks in 1942, during the transit of Supermarine Spitfires to Malta. The Imperial Japanese Navy design specification for what came to be the Japanese Mitsubishi A6M Zero fighter included endurance with drop tanks of two hours at full power, or six to eight hours at cruising speed. Drop tanks were commonly used with the Zero, even on combat air patrol (CAP). The Zero entered service in 1940. American bomber theorists (the so-called Bomber Mafia) wanted to create vast fleets of heavy bombers. They insisted formations of heavy bombers with multiple machine-gun positions would be self-defending, and that long-range escort fighters would be unnecessary, or even impossible to design. They were also concerned that long-range medium bombers might compete for resources. For all these reasons, official policy in the United States held that drop tanks should not be developed or encouraged. In the face of such entrenched attitudes in 1941 airmen such as Benjamin S. Kelsey and Oliver P. Echols worked quietly to get drop tank technology added to American fighters, beginning with the Lockheed P-38 Lightning. It was only with drop tanks supplying of extra fuel per fighter that P-38s could carry out Operation Vengeance, the downing of Admiral Isoroku Yamamoto's aircraft. (For this mission, each fighter carried one drop tank of approximately , and a larger one of approximately . Even after such experience showed the necessity for drop tanks, inflexible thinkers such as 8th Air Force General Ira C. Eaker had to be transferred out of commanding positions (and replaced with Maj. Gen. Jimmy Doolittle) so that drop tanks and range extension plans could be widely implemented in 1944 for American escort fighters. External drop tanks turned the Republic P-47 Thunderbolt from a short-range interceptor aircraft into a long-range escort and air superiority fighter, enabling it to accompany bombers from the British Isles into Germany and made it possible for heavy bomber formations to undertake daylight raids under escort by North American P-51 Mustangs. The P-38 could also carry two drop