The escort fighter was a concept for a fighter aircraft designed to escort bombers to and from their targets. 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 prior to the outbreak of World War II. Such heavy fighters largely failed in their intended escort role during the war, as they were commonly 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. The escort role has been diminished as modern air combat doctrine places a heavy emphasis on the idea of air superiority, and its importance in the ability of an air force to carry out effective operations. Air superiority is defined as a situation in which an air force dominates an airspace to such a degree as to be able to carry out any operations with no interference from enemy air combatants. Fighting an opponent with air superiority in a given battlespace is much harder, as any offensive or defensive tactics are likely to be overwhelmed. World War I and interwar period The first major strategic bombing campaigns were carried out during World War I. Initially using Zeppelins, and later by large bomber aircraft such as the Gotha G.IV and Handley Page Type O, these raids were increasingly countered by fighter aircraft. These fighters benefitted substantially from an inherent asymmetry; the attacking aircraft had to fly long distances to reach its target and thus had to be large enough to carry the required fuel load, while the defending fighters were flying only a short distance and therefore were able to be much lighter and have higher performance. In early 1916, the British War Office drew up a specification for a multi-seat escort fighter intended to protect formations of bombers from German fighters. While the specification did not require high speed, a good field of fire for its guns was essential, Prototype aircraft were built