Escort fighter
The escort fighter was a World War II concept for a fighter aircraft designed to escort bombers to and from their targets. Differing significantly in design from short-range, high-performance defensive fighters like the Supermarine Spitfire, escort fighters were usually inferior in a dogfight. The perfect escort fighter had long range, a lengthy combat loiter time to protect the bombers, and enough internal fuel to return home.
Use in the Luftwaffe
During the Battle of Britain, the Luftwaffe used Messerschmitt Bf 109s and Bf 110s based in France as escort fighter-bombers. Flying with the Heinkel He 111 and Junkers Ju 88 medium bombers, they would drop their light bomb-load and—for a brief period—fight off the British fighters. The Bf 109, however, was operating at the very edge of its range, and the Bf 110 had inferior performance; as escorts they provided only limited protection.
U.S. Army Air Corps operations
The U.S. Army Air Corps' precision strategic bombing campaign against German industries was only possible during the day. At first this was not seen as an issue; the Corps' newest bombers, the B-17 Flying Fortress and B-24 Liberator, were the most heavily-armed in history. Giant formations of them were planned, creating a crossfire of .50 caliber machine-guns that would fend off the enemy.
The plan was quickly undermined. Hitting a fast-moving fighter with guns in a turret proved extremely difficult; hitting a slow-moving bomber from a fighter with a gunsight was much easier. USAAF bomber losses gradually increased, and experimental "gunships" like the YB-40 did nothing to reduce them.
It was not until the introduction of the P-38 Lightning and P-47 Thunderbolt that the bombing raids could claim a measure of success. Able to carry large Lockheed-designed drop tanks, the fighters were able to escort the bombers for much of their missions. The first Allied fighters over Berlin were 55FG P-38s on March 3 1944. When the Merlin-powered P-51 Mustang was introduced, with a laminar-flow wing for efficiency, the final escort fighter development of the war was complete. The combination of strategic bombing and masses of powerful escort fighters allowed the USAAF to open a "second front in the air" against Germany long before a ground campaign was possible.
Post-war
The successes of the P-47 and P-51 gave the impression that the escort fighter was a concept worth continuing after the end of the war. The high fuel use of early jet engines made such aircraft difficult to design, and a number of experimental designs were tried that used mixed power, typically a turboprop and jet, but these failed to meet performance requirements. A new concept, the XF-85 Goblin microfighter, planned to act as a parasite fighter for the Convair B-36, was tested with a B-29 Superfortress and found to be utterly impossible to use operationally. Later the FICON project attempted a similar solution, docking jet fighters with heavy bombers via a trapeze mechanism or their wingtips.
The XB-70 Valkyrie, North American Aviation's Mach 3 bomber, was intended to be immune to enemy attack as a result of speed. When the Soviets development of the Mach 3 MiG-25 interceptor became known, however, NAA began work on the XF-108 Rapier. The Rapier was intended to protect the XB-70, using many of the same technologies; when the bomber was cancelled, so too was the XF-108.
The Modern Era
With the advancement of technology, the role of the dedicated escort fighter was gradually merged with other fighter types, so the term fell out of use. In Vietnam, escort for American bombers were provided normally by F-4 Phantom IIs and sometimes F-8 Crusaders in the Navy. With the advent of the Air Superiority Fighter, F-15 Eagles took on this role in the 1991 Gulf War.
Still, during the Vietnam War, a variation on the escort fighter was also developed to suppress enemy missile defenses. The SEAD (suppression of enemy air defenses) role was given to F-100 Super Sabre and later F-105 Thunderchief and F-4 Phantom II fighters, nicknamed Wild Weasels, which would strike during or just before a raid.