The term "heavy fighter" is one that a number of historians and analysts have applied ex post facto, but it was not one every air force used in the 1930s and 40s. Generally speaking, heavy fighters were large, twin-engined aircraft, with a heavy armament, usually some form of autocannon. But just because these aircraft shared the same features does not mean that air forces used them in the same fashion. For example, the IJA developed the Kawasaki Ki-45 as a bomber escort while the French conceived of the Potez 63 more as an interceptor. The Luftwaffe's Zerstörer concept envisioned long-ranged fighters making broad sweeps over enemy territory as an offensive weapon. The heavy fighters as a whole had a rather disappointing war record with few exceptions. But this failure sometimes obscures the rationale why many air forces adapted these types.

One of the critical advantages of a heavy fighter is that the combined power of its two engines allowed it to carry aloft a bigger weight. Not only did this include fuel, which gave many heavy fighters a good deal of range, but also weapons. The development of autocannon proceeded apace in the 1920s and 30s and while weights went down, autocannon of 15mm caliber and above were still quite bulky and heavy. Not only did aircraft have to carry the weight of these autocannon, but also the attending weight of the mountings that absorbed recoil and the ammunition for such guns. This meant that carrying such weapons came along with a not insignificant weight-penalty.

There were two other related problems for autocannon that faced aviation designers in the 1930s that led them to favor twin-engined designs. For one, mounting heavy autocannon on single-engined planes was difficult. There were roughly two options for a single-engine autocannon: the wings or through the centerline. Wing-mounted cannons were limited not only by the amount of ammunition they could carry, but also because the wight of the cannon and their mountings could damage the integrity of the wings. Wing-mounted cannons also flexed while firing, which diminished accuracy. The centerline option was better, but likewise limited the designers to inline engines. Furthermore, the autocannon's caliber was limited by engine size; some engines like the German DB 600 series could incorporate enough spare room within the engine for an autocannon to fire through, but not all inline engines could accomplish this. Placing the engines in the wing and concentrating firepower in the nose was a solution to these design problems.

The second factor was that the heavy fighter nose-mounted armament allowed for the concentration of fire. This made it very attractive to designers in that it meant that this fighter could realize the full potential of heavy-caliber armament. Although this video is of a late-war 30 mm autocannon with special ammunition, it does illustrate the potential destructive power of these weapons. Not only could heavy fighters field these weapons, but they could carry more ammunition than single-engined types and bring them to bear on an enemy aircraft. Nose-mounted armament was less prone to fly off course due to the trajectory of the firing aircraft than wing-mounted guns and it was also easier to deflection shoot as the guns were more or less within the center axis of the plane (ie the part of the plane that was moving the least).

This was one of the reasons why many heavy fighters were favored as bomber destroyers by WWII air forces. This however was not one of the initial goals many aviation designers intended for their heavy fighters. The Bf-110 was the first German Zerstörer and while its early war use saw it fielded as an offensive aircraft, it increasingly became a bomber destroyer and night fighter by the mid-war. The range of the 110 allowed it to loiter over the Reich and it could carry new equipment like radar or underwing rockets and still maintain interception speed and ceiling.

The Bf 110 though does have a somewhat undeserved reputation as a failure that some popular aviation historians ascribe to the heavy fighter concept as a whole. Although popular historical accounts of the Battle of Britain emphasize how the escort fighter needed escorts, the data from most Zerstörer groups show that they had a more mixed record. While they certainly could not engage in a scrap with the more nimble RAF fighters, the 110s often performed their jobs and did not suffer catastrophic losses over the UK. The Bf 110 was certainly not an unmitigated failure like the Italian Breda Ba.88, but it was also not quite the sterling success the Luftwaffe had hoped either.

Part of the problem for the heavy fighter concept as a whole was that its gestation coincided with a massive development of aerial technology. What was a top of the line engine in 1937, for instance, could soon become uncompetative as engines became more powerful and boosted by developments like water-methanol injection. There were also new weight-saving measures like new duraaluminum alloys that could provide stronger airframes at less weight. Single-engined fighters became far more capable in the 1940s than they were in the prior decade. The USAAF's quest for a long-range air escort fighter in 1942 had the advantage that they had airframes and engines that could allow for greater amounts of fuel to be carried. So the heavy fighters suddenly had to compete with single-engined aircraft for the same roles such as bomber escorts. Designers in the 1930s had greater difficulty creating single-engined types to meet these roles, their wartime counterparts did not.

There were also tactical reasons for the failures of the heavy fighter. For one thing, the large aircraft created a massive visual signature in the sky. For all the romance of fighter combat in popular culture, more often than not aerial victories come from surprise as the victor was often able to position themselves in an advantageous position. Heavy fighters may have had advantages in speed and armament, but they were hard to achieve surprise. Hub Zemke, who flew P-38s, -51s, and 47s, noted this in his evaluation of the P-38:

Another factor to degrade the P-38's combat capability was the identification factor. The eyes of a pilot often picked up specks in the distance that could not be immediately identified as friend or foe. These were reported in as "bogies." Appropriate tactical maneuvers were taken to prevent bogies from having the advantage of a subsequent attack. In the case of the P-38 the twin booms and slab elevator gave this aircraft's identity away- as far as the eye could see.

This was compounded by the fact that the advent of radar and other defense networks made the type of offensive sweeps embodied by the Zerstörer somewhat more difficult to conduct.

Overall, the heavy fighter was not an entirely fool-hardy diversion or caprice on the part of air forces prior to the Second World War. The heavy fighters as a whole filled in niches that proved to be quite valuable in the air war. Had the war broken out sooner, some heavy fighters might have given a better accounting of themselves as it was more difficult in this period to make single-engined aircraft perform these roles. Timing though, was everything and most of the heavy fighters used during the war tended to be developed too early for a later air war. This had the effect of eclipsing the heavy fighters' virtues and enhanced their vices.