A heavy fighter is a fighter aircraft designed to carry heavier weapons or operate at longer ranges. To achieve acceptable performance, most heavy fighters were twin-engined, and many had multi-place crews.
The twin-engine heavy fighter was a major design class during the pre-World War II period, conceived as long-range escort fighters or heavily armed bomber destroyers. With the exception of the Lockheed P-38 Lightning and the de Havilland Mosquito, heavy fighters largely failed in their intended roles during World War II, as they could not outmaneuver the more conventional, single-engined fighters. Many twin-engined heavy fighters eventually found their niche as night fighters, with considerable successes.
A major heavy fighter design was the Messerschmitt Bf 110, a German fighter that, prior to the war, the Luftwaffe considered more important than their single-engine fighters. Many of the best pilots were assigned to Bf 110 wings, and were specifically designated as Zerstörergeschwader ("destroyer") wings. While lighter fighters were intended for defense, the destroyers were intended for offensive missions: to escort bombers on missions at long range, then use its superior speed to outrun defending fighters that would be capable of outmaneuvering it.
This doctrine proved to be a costly mistake. In practice the Bf 110 was capable of using this combination of features for only a short time, until the late summer of 1940. It served well against the Hawker Hurricane during the Battle of France, but was easily outperformed by – and up to 50 km/h (31 mph) slower in top speed than – the Supermarine Spitfire during the Battle of Britain. Eventually Bf 110s were converted to interceptors, and were particularly successful in the later marks of the Bf 110G series from 1942/3 onwards as night fighters, serving as the primary aircraft of multiple Luftwaffe Nachtjagdgeschwader night fighter wings, using multiple versions of the Lichtenstein radar for nocturnal interception of RAF Bomber Command heavy bombers, as well as finding some use as ground-attack aircraft. The Me 210 and Me 410 Hornisse were all-new aircraft designs meant to replace the Bf 110, but also could not outrun contemporary single-engine fighters, with the Me 210 having serious aerodynamic problems from mistakes in the design of its wing planform and the initial design of its rear fuselage.