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. In the post-war era the introduction of jet engines and their inherent short range made escort fighters very difficult to build. The related concept of a penetration fighter emerged briefly in the 1950s and again in the 1960s, but did not result in any production aircraft.