A drop tank is an expendable fuel tank carried by an aircraft, usually on an external hard point. It allows for longer operating times, leading to either increased range or increased loiter time. Typically, when the fuel carried within a drop tank is gone, the pilot can jettison the drop tank (hence its name) to reduce weight and drag on the aircraft. Drop tanks first appeared (with reliable records) on World War II aircraft during the Battle of Britain. Some Luftwaffe fighters lacked the range to reach the United Kingdom from inland airbases, rendering German bombers vulnerable without fighter escorts. Focke-Wulf and Messerschmitt escort fighters were fitted with drop tanks to increase their operating time over English targets. Later on in the war, as Allied bombers ranged over continental Europe, drop tanks were fitted to formerly-defensive fighters such as the Supermarine Spitfire, Hawker Hurricane and Republic P-47 Thunderbolt to allow them to escort long-range bombers such as the Boeing B-17 Flying Fortress and Avro Lancaster all the way to their targets.
Drop tanks had some effect on fighter tactics. Since a fighter carrying a drop tank (especially a full one) is much clumsier than one without, engaging a laden fighter will almost always force it to jettison its drop tank in order to defend itself. As a result, if the defenders met the attackers at the end of a long flight on the latter's part (such as the RAF in the early phases, and the Luftwaffe in the later) they could attempt to intercept enemy fighters as far out as possible even in inferior numbers. Once the attackers dropped their tanks, their endurance would be reduced; they might even be forced to head for home. This is why fuel in the drop tank is always used first!