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.