Fighter aircraft
The period of improving the same biplane design over and over was now coming to an end, and the Hawker Hurricane and Supermarine Spitfire started to supplant the Gloster Gladiator and Hawker Fury biplanes but many biplanes remained in front-line service well past the start of World War II. While not a combatant in Spain, they too absorbed many of the lessons in time to use them.
The Spanish Civil War also provided an opportunity for updating fighter tactics. One of the innovations was the development of the "finger-four" formation by the German pilot Werner Mölders. Each fighter squadron (German: Staffel) was divided into several flights (Schwärme) of four aircraft. Each Schwarm was divided into two Rotten, which was a pair of aircraft. Each Rotte was composed of a leader and a wingman. This flexible formation allowed the pilots to maintain greater situational awareness, and the two Rotten could split up at any time and attack on their own. The finger-four would be widely adopted as the fundamental tactical formation during World War Two, including by the British and later the Americans.

1939–1945: World War II: World War II featured fighter combat on a larger scale than any other conflict to date. German Field Marshal Erwin Rommel noted the effect of airpower: "Anyone who has to fight, even with the most modern weapons, against an enemy in complete command of the air, fights like a savage…"  Throughout the war, fighters performed their conventional role in establishing air superiority through combat with other fighters and through bomber interception, and also often performed roles such as tactical air support and reconnaissance.
Fighter design varied widely among combatants. The Japanese and Italians favored lightly armed and armored but highly maneuverable designs such as the Japanese Nakajima Ki-27, Nakajima Ki-43 and Mitsubishi A6M Zero and the Italian Fiat G.50 Freccia and Macchi MC.200. In contrast, designers in the United Kingdom, Germany, the Soviet Union, and the United States believed that the increased speed of fighter aircraft would create g-forces unbearable to pilots who attempted maneuvering dogfights typical of the First World War, and their fighters were instead optimized for speed and firepower.  In practice, while light, highly maneuverable aircraft did possess some advantages in fighter-versus-fighter combat, those could usually be overcome by sound tactical doctrine, and the design approach of the Italians and Japanese made their fighters ill-suited as interceptors or attack aircraft.

European theater: During the invasion of Poland and the Battle of France, Luftwaffe fighters—primarily the Messerschmitt Bf 109—held air superiority, and the Luftwaffe played a major role in German victories in these campaigns.  During the Battle of Britain, however, British Hurricanes and Spitfires proved roughly equal to Luftwaffe fighters.  Additionally Britain's radar-based Dowding system directing fighters onto German attacks and the advantages of fighting above Britain's home territory allowed the RAF to deny Germany air superiority, saving the UK from possible German invasion and dealing the Axis a major defeat early in the Second World War.  On the Eastern Front, Soviet fighter forces were overwhelmed during the opening phases of Operation Barbarossa.  This was a result of the tactical surprise at the outset of the campaign, the leadership vacuum within the Soviet military left by the Great Purge, and the general inferiority of Soviet designs at the time, such as the obsolescent Polikarpov I-15 biplane and the I-16. More modern Soviet designs, including the Mikoyan-Gurevich MiG-3, LaGG-3 and Yakolev Yak-1, had not yet arrived in numbers and in any case were still inferior to the Messerschmitt Bf 109. As a result, during the early months of these campaigns, Axis air forces destroyed large numbers of Red Air Force aircraft on the ground and in one-sided dogfights.  In the later stages on the Eastern Front, Soviet training and leadership improved, as did their equipment. By 1942 Soviet designs such as the Yakovlev Yak-9 and Lavochkin La-5 had performance comparable to the German Bf 109 and Focke-Wulf Fw 190. Also, significant numbers of British, and later U.S., fighter aircraft were supplied to aid the Soviet war effort as part of Lend-Lease, with the Bell P-39 Airacobra proving particularly effective in the lower-altitude combat typical of the Eastern Front. The Soviets were also helped indirectly by the American and British bombing campaigns, which forced the Luftwaffe to shift many of its fighters away from the Eastern Front in defense against these raids.  The Soviets increasingly were able to challenge the Luftwaffe, and while the Luftwaffe maintained a qualitative edge over the Red Air Force for much of the war, the increasing numbers and efficacy of the Soviet Air Force were critical to the Red Army's efforts at turning back and eventually annihilating the Wehrmacht.

Meanwhile, air combat on the Western Front had a much different character.  Much of this combat focused on the strategic bombing campaigns of the RAF and the USAAF against German industry intended to wear down the Luftwaffe.  Axis fighter aircraft focused on defending against Allied bombers while Allied fighters' main role was as bomber escorts.  The RAF raided German cities at night, and both sides developed radar-equipped night fighters for these battles.  The Americans, in contrast, flew daylight bombing raids into Germany delivering the Combined Bomber Offensive.  Unescorted Consolidated B-24 Liberators and Boeing B-17 Flying Fortress bombers, however, proved unable to fend off German interceptors (primarily Bf 109s and Fw 190s).  With the later arrival of long range fighters, particularly the North American P-51 Mustang, American fighters were able to escort far into Germany on daylight raids and by ranging ahead attrited the Luftwaffe to establish control of the skies over Western Europe.
By the time of Operation Overlord in June 1944, the Allies had gained near complete air superiority over the Western Front.  This cleared the way both for intensified strategic bombing of German cities and industries, and for the tactical bombing of battlefield targets.  With the Luftwaffe largely cleared from the skies, Allied fighters increasingly served as ground attack aircraft.
Allied fighters, by gaining air superiority over the European battlefield, played a crucial role in the eventual defeat of the Axis, which Reichmarshal Hermann Göring, commander of the German Luftwaffe summed up when he said: "When I saw Mustangs over Berlin, I knew the jig was up."

Pacific theater: Major air combat during the war in the Pacific began with the entry of the Western Allies following Japan's attack against Pearl Harbor.  The Imperial Japanese Navy Air Service primarily operated the Mitsubishi A6M Zero, and the Imperial Japanese Army Air Service flew the Nakajima Ki-27 and the Nakajima Ki-43, initially enjoying great success, as these fighters generally had better range, maneuverability, speed and climb rates than their Allied counterparts. Additionally, Japanese pilots were well trained and many were combat veterans from Japan's campaigns in China.  They quickly gained air superiority over the Allies, who at this stage of the war were often disorganized, under-trained and poorly equipped, and Japanese air power contributed significantly to their successes in the Philippines, Malaysia and Singapore, the Dutch East Indies and Burma.
By mid-1942, the Allies began to regroup and while some Allied aircraft such as the Brewster Buffalo and the P-39 Airacobra were hopelessly outclassed by fighters like Japan's Mitsubishi A6M Zero, others such as the Army's Curtiss P-40 Warhawk and the Navy's Grumman F4F Wildcat possessed attributes such as superior firepower, ruggedness and dive speed, and the Allies soon developed tactics (such as the Thach Weave) to take advantage of these strengths.  These changes soon paid dividends, as the Allied ability to deny Japan air superiority was critical to their victories at Coral Sea, Midway, Guadalcanal and New Guinea.  In China, the Flying Tigers also used the same tactics with some success, although they were unable to stem the tide of Japanese advances there.
By 1943, the Allies began to gain the upper hand in the Pacific Campaign's air campaigns.  Several factors contributed to this shift.  First, the Lockheed P-38 Lightning and second-generation Allied fighters such as the Grumman F6 Hellcat and later the Vought F4 Corsair, the Republic P-47 Thunderbolt and the North American P-51 Mustang, began arriving in numbers.  These fighters outperformed Japanese fighters in all respects except maneuverability.  Other problems with Japan's fighter aircraft also became apparent as the war progressed, such as their lack of armor and light armament, which had been typical of all pre-war fighters worldwide, but the problem was particularly difficult to rectify on the Japanese designs. 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'.