An escort fighter needed range long enough to reach the target, loiter over it for the duration of the raid to defend the bombers, and return. A number of twin-engined heavy fighters with high fuel capacity were designed for escort duties before World War II. Such heavy fighters largely failed in their intended escort role during the war, as they were outmaneuvered by more agile single-engined fighters. As the war progressed, longer-range fighter designs and the use of drop tanks allowed single-engined fighters to perform escort duties.