Fighter aircraft
This made them inadequate as either bomber-interceptors or ground-attack aircraft, roles Allied fighters were still able to fill. Most importantly, Japan's training program failed to provide enough well-trained pilots to replace losses.  In contrast, the Allies improved both the quantity and quality of pilots graduating from their training programs.  By mid-1944, Allied fighters had gained air superiority throughout the theater, which would not be contested again during the war.  The extent of Allied quantitative and qualitative superiority by this point in the war was demonstrated during the Battle of the Philippine Sea, a lopsided Allied victory in which Japanese fliers were shot down in such numbers and with such ease that American fighter pilots likened it to a great 'turkey shoot'.  Late in the war, Japan began to produce new fighters such as the Nakajima Ki-84 and the Kawanishi N1K to replace the Zero, but only in small numbers, and by then Japan lacked the trained pilots or sufficient fuel to mount an effective challenge to Allied attacks.  During the closing stages of the war, Japan's fighter arm could not seriously challenge raids over Japan by American Boeing B-29 Superfortresses, and was largely reduced to Kamikaze attacks.

Technological innovations: Fighter technology advanced rapidly during the Second World War. Piston-engines, which powered the vast majority of World War II fighters, grew more powerful: at the beginning of the war fighters typically had engines producing between 1,000 hp (750 kW)  and 1,400 hp (1,000 kW), while by the end of the war many could produce over 2,000 hp (1,500 kW).  For example, the Spitfire, one of the few fighters in continuous production throughout the war, was in 1939 powered by a 1,030 hp (770 kW) Merlin II, while variants produced in 1945 were equipped with the 2,035 hp (1,517 kW) Rolls-Royce Griffon 61. Nevertheless, these fighters could only achieve modest increases in top speed due to problems of compressibility created as aircraft and their propellers approached the sound barrier, and it was apparent that propeller-driven aircraft were approaching the limits of their performance. German jet and rocket-powered fighters entered combat in 1944, too late to impact the war's outcome. The same year the Allies' only operational jet fighter, the Gloster Meteor, also entered service. World War II fighters also increasingly featured monocoque construction, which improved their aerodynamic efficiency while adding structural strength. Laminar flow wings, which improved high speed performance, also came into use on fighters such as the P-51 Mustang, while the Messerschmitt Me 262 and the Messerschmitt Me 163 featured swept wings that dramatically reduced drag at high subsonic speeds.  Armament also advanced during the war.  The rifle-caliber machine guns that were common on prewar fighters could not easily down the more rugged warplanes of the era. Air forces began to replace or supplement them with cannons, which fired explosive shells that could blast a hole in an enemy aircraft – rather than relying on kinetic energy from a solid bullet striking a critical component of the aircraft, such as a fuel line or control cable, or the pilot. Cannons could bring down even heavy bombers with just a few hits, but their slower rate of fire made it difficult to hit fast-moving fighters in a dogfight.  Eventually, most fighters mounted cannons, sometimes in combination with machine guns.  The British epitomized this shift. Their standard early war fighters mounted eight .303 in (7.7 mm) caliber machine guns, but by mid-war they often featured a combination of machine guns and 20 mm (0.79 in) cannons, and late in the war often only cannons. The Americans, in contrast, had problems producing a cannon design, so instead placed multiple .50 in (12.7 mm) heavy machine guns on their fighters.  Fighters were also increasingly fitted with bomb racks and air-to-surface ordnance such as bombs or rockets beneath their wings, and pressed into close air support roles as fighter-bombers. Although they carried less ordnance than light and medium bombers, and generally had a shorter range, they were cheaper to produce and maintain and their maneuverability made it easier for them to hit moving targets such as motorized vehicles. Moreover, if they encountered enemy fighters, their ordnance (which reduced lift and increased drag and therefore decreased performance) could be jettisoned and they could engage enemy fighters, which eliminated the need for fighter escorts that bombers required.
Heavily armed fighters such as Germany's Focke-Wulf Fw 190, Britain's Hawker Typhoon and Hawker Tempest, and America's Curtiss P-40, F4U Corsair, P-47 Thunderbolt and P-38 Lightning all excelled as fighter-bombers, and since the Second World War ground attack has become an important secondary capability of many fighters.
World War II also saw the first use of airborne radar on fighters.  The primary purpose of these radars was to help night fighters locate enemy bombers and fighters.  Because of the bulkiness of these radar sets, they could not be carried on conventional single-engined fighters and instead were typically retrofitted to larger heavy fighters or light bombers such as Germany's Messerschmitt Bf 110 and Junkers Ju 88, Britain's de Havilland Mosquito and Bristol Beaufighter, and America's Douglas A-20, which then served as night fighters.  The Northrop P-61 Black Widow, a purpose-built night fighter, was the only fighter of the war that incorporated radar into its original design. Britain and America cooperated closely in the development of airborne radar, and Germany's radar technology generally lagged slightly behind Anglo-American efforts, while other combatants developed few radar-equipped fighters.
A concept originated from German engineer Bernhard J. Schrage in 1943 as a response to the increasing threat posed by Allied heavy bombers, particularly at night. The Schrage Musik system involved mounting upward-facing cannon turrets, typically twin 20mm or 30mm guns, in the belly of German night fighters such as the Messerschmitt Bf 110 and later versions of the Junkers Ju 88. These guns were angled upwards to target the vulnerable underside of enemy bombers.

1946–present: Post–World War II period: Several prototype fighter programs begun early in 1945 continued on after the war and led to advanced piston-engine fighters that entered production and operational service in 1946. A typical example is the Lavochkin La-9 'Fritz', which was an evolution of the successful wartime Lavochkin La-7 'Fin'. Working through a series of prototypes, the La-120, La-126 and La-130, the Lavochkin design bureau sought to replace the La-7's wooden airframe with a metal one, as well as fit a laminar flow wing to improve maneuver performance, and increased armament. The La-9 entered service in August 1946 and was produced until 1948; it also served as the basis for the development of a long-range escort fighter, the La-11 'Fang', of which nearly 1200 were produced 1947–51. Over the course of the Korean War, however, it became obvious that the day of the piston-engined fighter was coming to a close and that the future would lie with the jet fighter.
This period also witnessed experimentation with jet-assisted piston engine aircraft. La-9 derivatives included examples fitted with two underwing auxiliary pulsejet engines (the La-9RD) and a similarly mounted pair of auxiliary ramjet engines (the La-138); however, neither of these entered service. One that did enter service – with the U.S. Navy in March 1945 – was the Ryan FR-1 Fireball; production was halted with the war's end on VJ-Day, with only 66 having been delivered, and the type was withdrawn from service in 1947. The USAAF had ordered its first 13 mixed turboprop-turbojet-powered pre-production prototypes of the Consolidated Vultee XP-81 fighter, but this program was also canceled by VJ Day, with 80% of the engineering work completed.

Rocket-powered fighters: The first rocket-powered aircraft was the Lippisch Ente, which made a successful maiden flight in March 1928. The only pure rocket aircraft ever mass-produced was the Messerschmitt Me 163B Komet in 1944, one of several German World War II projects aimed at developing high speed, point-defense aircraft. Later variants of the Me 262 (C-1a and C-2b) were also fitted with "mixed-power" jet/rocket powerplants, while earlier models were fitted with rocket boosters, but were not mass-produced with these modifications.
The USSR experimented with a rocket-powered interceptor in the years immediately following World War II, the Mikoyan-Gurevich I-270. Only two were built.
In the 1950s, the British developed mixed-power jet designs employing both rocket and jet engines to cover the performance gap that existed in turbojet designs. The rocket was the main engine for delivering the speed and height required for high-speed interception of high-level bombers and the turbojet gave increased fuel economy in other parts of flight, most notably to ensure the aircraft was able to make a powered landing rather than risking an unpredictable gliding return.