A drop tank is an auxiliary fuel tank used on military fighting airplanes to extend their flying range. These belly tanks, as they were commonly called, were carried under the airplane and were easily jettisoned once the fighting began. The belly tank was released from the airplane when the pilot flipped a switch inside the cockpit. The fuel tank dropped away from the plane, hence the name drop tank. If the tank was not released, it became an attractive target for enemy pilots who could easily blow up the plane by hitting the exposed fuel tank.
In World War II, bombing missions were the key to victory for both the Nazis and the Allies. Bombers were big, slow and very vulnerable to enemy fighters. In an effort to protect the bombers, both sides needed to design a plane that would allow protective fighter plane coverage for bombers. The problem was that the bombers could fly much deeper into enemy air space than the fuel supply of the fighters would allow. The result was the advent of the drop tank.
By designing a fuel tank that could be carried under the fuselage of the fighter, its range could be drastically extended. This added range allowed the fighters to accompany bombers deep into enemy air space and battle enemy fighter aircraft that attempted to shoot down the bombers. The danger lay in the fuel tank's exposure to enemy fire. A single bullet could cause a dangerous explosion that would destroy the fighter plane.
A mechanism was developed that allowed the fuel tanks to be dropped out of the way once a battle had begun. This new drop tank gave the fighter plane added flying distance and an easy removal when the fighting started. The benefit of fighter protection soon added increased drop tank usage by every branch of the air services on both sides of the war. The fighter drop tank soon found a following in the civilian population of the United States.
Salt flat speed racers adopted the drop tank for aerodynamic race car bodies. The drop tanks became known as belly tankers and were all the rage in the 1950's speed races. Many world land speed records were set using a Flathead Ford V-8 mounted in a surplus Army drop tank. Many records set by those land-based belly tanks remained unbroken for years. Ultimately, space age designs replaced the vintage drop tank designs and set records of their own.