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, 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. Only the Northrop P-61 Black Widow of the USAAF was ever built from the start, during the World War II era, solely to be a night fighter.
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 squadrons, and they were specifically designated as Zerstörergeschwader ("destroyer") wings. While other, lighter, fighters were mainly intended for defense, the destroyers were the ones mainly 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 only capable of using this combination of features for 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 - 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.
Towards the end of the war, the Dornier Do 335 Pfeil could have been an ideal twin-engined Zerstörer fighter design for the Luftwaffe due to its centre line thrust format, which placed its fuselage-mounted twin DB 603 engines' propellers on opposing ends of the fuselage, and potentially allowed much better maneuverability, and did allow dramatically higher speeds (just over 750 km/h or 465 mph), than any other twin-piston-engined aircraft of its era, but like so many other advanced German aircraft designs that were actually produced by Germany late in the war, the Do 335 never had the chance to be produced in quantity.
Following the example set by the Bf 110, the Japanese built the broadly similar Kawasaki Ki-45 Toryu. Likewise neutral Netherlands built the twin-boom Fokker G.I, only to be seized by the Luftwaffe after the German invasion of the Netherlands.
Perhaps in the belief that "The bomber will always get through", the British lagged behind in its heavy fighter development. Apart from the Westland Whirlwind and Welkin, built only in modest numbers, the Royal Air Force's wartime heavy fighters were all adapted from earlier bombers. During the Battle of Britain, Bristol Blenheim bombers were fitted, as an interim measure and in utmost secrecy, with radars and ventral gun packs, turning them into the RAF's first night fighters.
More successful was the Bristol Beaufighter, which reused major portions of the earlier Beaufort torpedo bomber. Armed with six .303 inch machine guns, four 20 mm cannon and an assortment of bombs and rockets, the Beaufighter was potent in the anti-ship and ground attack role in the Pacific and Europe. With the addition of radar, it was one of the Royal Air Force's main night fighters. Similarly, the de Havilland Mosquito fast bomber was later adapted for both day and night fighter use.
United States 
During the late 1930s, Bell Aircraft of the United States designed the YFM-1 Airacuda "bomber destroyer". Very large and heavily armed, the Airacuda was plagued with design flaws; only 13 examples were eventually built, none of which participated in WWII.
The most successful heavy fighter of the war was the Lockheed P-38 Lightning. It was designed to carry heavy armament at high speed or long range. For a variety of reasons, notably its excellent twin General Electric-designed turbochargers and its crew of one (rather than two or three), it dramatically outperformed its German and British counterparts. In service it was used as an escort fighter, following B-17 Flying Fortress raids deep into German-held Europe where it was able to hold its own with the much lighter German fighters. In its escort role, the P-38 was the first Allied fighter over Berlin. It was also highly successful in the Pacific theatre, where its long range proved a pivotal advantage. Expensive to produce and maintain, it was relegated to other roles when the single-engined, but equally long-ranged, P-51D Mustang reached squadrons.
The last piston-engined heavy fighters to see service were the de Havilland Hornet and Sea Hornet, and the F-82 Twin Mustang. All were developed at the end of World War II for use in the Pacific theatre, though none reached operational squadrons until after VJ day, the Hornet in 1946, the Sea Hornet in 1947 and the Twin Mustang in 1948.
Although numerous modern fighters could be called "heavy", with regard to their weight, the term is generally no longer used, especially with the era of piston-engined front line fighter aircraft in any size having ended in the mid-20th century.
- Weal, John (1999). Messerschmitt Bf 110 Zerstörer aces of World War 2. Oxford: Osprey Aviation. pp. 6–7. ISBN 1-85532-753-8.
- Murphy, Justin D.; McNiece, Matthew A. (2009). Military aircraft, 1919-1945: an illustrated history of their impact (1st ed. ed.). Santa Barbara: ABC-CLIO. ISBN 1-85109-498-9.
- Mr Baldwin on Aerial Warfare - A Fear For The Future. The Times newspaper, 11 November 1932 p7 column B.
See also