Fighter aircraft
The success of the Eindecker kicked off a competitive cycle of improvement among the combatants, both sides striving to build ever more capable single-seat fighters. The Albatros D.I and Sopwith Pup of 1916 set the classic pattern followed by fighters for about twenty years. Most were biplanes and only rarely monoplanes or triplanes. The strong box structure of the biplane provided a rigid wing that allowed the accurate control essential for dogfighting. They had a single operator, who flew the aircraft and also controlled its armament. They were armed with one or two Maxim or Vickers machine guns, which were easier to synchronize than other types, firing through the propeller arc. Gun breeches were in front of the pilot, with obvious implications in case of accidents, but jams could be cleared in flight, while aiming was simplified.

The use of metal aircraft structures was pioneered before World War I by Breguet but would find its biggest proponent in Anthony Fokker, who used chrome-molybdenum steel tubing for the fuselage structure of all his fighter designs, while the innovative German engineer Hugo Junkers developed two all-metal, single-seat fighter monoplane designs with cantilever wings: the strictly experimental Junkers J 2 private-venture aircraft, made with steel, and some forty examples of the Junkers D.I, made with corrugated duralumin, all based on his experience in creating the pioneering Junkers J 1 all-metal airframe technology demonstration aircraft of late 1915.  While Fokker would pursue steel tube fuselages with wooden wings until the late 1930s, and Junkers would focus on corrugated sheet metal, Dornier was the first to build a fighter (the Dornier-Zeppelin D.I) made with pre-stressed sheet aluminum and having cantilevered wings, a form that would replace all others in the 1930s.  As collective combat experience grew, the more successful pilots such as Oswald Boelcke, Max Immelmann, and Edward Mannock developed innovative tactical formations and maneuvers to enhance their air units' combat effectiveness.
Allied and – before 1918 – German pilots of World War I were not equipped with parachutes, so in-flight fires or structural failures were often fatal. Parachutes were well-developed by 1918 having previously been used by balloonists, and were adopted by the German flying services during the course of that year. The well known and feared Manfred von Richthofen, the "Red Baron", was wearing one when he was killed, but the allied command continued to oppose their use on various grounds.
In April 1917, during a brief period of German aerial supremacy a British pilot's average life expectancy was calculated to average 93 flying hours, or about three weeks of active service. More than 50,000 airmen from both sides died during the war.

1919–1938: Inter-war period: Fighter development stagnated between the wars, especially in the United States and the United Kingdom, where budgets were small. In France, Italy and Russia, where large budgets continued to allow major development, both monoplanes and all metal structures were common. By the end of the 1920s, however, those countries overspent themselves and were overtaken in the 1930s by those powers that hadn't been spending heavily, namely the British, the Americans, the Spanish (in the Spanish civil war) and the Germans.
Given limited budgets, air forces were conservative in aircraft design, and biplanes remained popular with pilots for their agility, and remained in service long after they ceased to be competitive. Designs such as the Gloster Gladiator, Fiat CR.42 Falco, and Polikarpov I-15 were common even in the late 1930s, and many were still in service as late as 1942. Up until the mid-1930s, the majority of fighters in the US, the UK, Italy and Russia remained fabric-covered biplanes.
Fighter armament eventually began to be mounted inside the wings, outside the arc of the propeller, though most designs retained two synchronized machine guns directly ahead of the pilot, where they were more accurate (that being the strongest part of the structure, reducing the vibration to which the guns were subjected). Shooting with this traditional arrangement was also easier because the guns shot directly ahead in the direction of the aircraft's flight, up to the limit of the guns range; unlike wing-mounted guns which to be effective required to be harmonised, that is, preset to shoot at an angle by ground crews so that their bullets would converge on a target area a set distance ahead of the fighter. Rifle-caliber .30 and .303 in (7.62 and 7.70 mm) calibre guns remained the norm, with larger weapons either being too heavy and cumbersome or deemed unnecessary against such lightly built aircraft. It was not considered unreasonable to use World War I-style armament to counter enemy fighters as there was insufficient air-to-air combat during most of the period to disprove this notion.

 
The rotary engine, popular during World War I, quickly disappeared, its development having reached the point where rotational forces prevented more fuel and air from being delivered to the cylinders, which limited horsepower. They were replaced chiefly by the stationary radial engine though major advances led to inline engines gaining ground with several exceptional engines—including the 1,145 cu in (18,760 cm3) V-12 Curtiss D-12. Aircraft engines increased in power several-fold over the period, going from a typical 180 hp (130 kW) in the 900 kg (2,000 lb) Fokker D.VII of 1918 to 900 hp (670 kW) in the 2,500 kg (5,500 lb) Curtiss P-36 of 1936. The debate between the sleek in-line engines versus the more reliable radial models continued, with naval air forces preferring the radial engines, and land-based forces often choosing inlines. Radial designs did not require a separate (and vulnerable) radiator, but had increased drag. Inline engines often had a better power-to-weight ratio.
Some air forces experimented with "heavy fighters" (called "destroyers" by the Germans). These were larger, usually twin-engined aircraft, sometimes adaptations of light or medium bomber types. Such designs typically had greater internal fuel capacity (thus longer range) and heavier armament than their single-engine counterparts. In combat, they proved vulnerable to more agile single-engine fighters.
The primary driver of fighter innovation, right up to the period of rapid re-armament in the late 1930s, were not military budgets, but civilian aircraft racing. Aircraft designed for these races introduced innovations like streamlining and more powerful engines that would find their way into the fighters of World War II. The most significant of these was the Schneider Trophy races, where competition grew so fierce, only national governments could afford to enter.
At the very end of the inter-war period in Europe came the Spanish Civil War. This was just the opportunity the German Luftwaffe, Italian Regia Aeronautica, and the Soviet Union's Voenno-Vozdushnye Sily needed to test their latest aircraft. Each party sent numerous aircraft types to support their sides in the conflict. In the dogfights over Spain, the latest Messerschmitt Bf 109 fighters did well, as did the Soviet Polikarpov I-16. The later German design was earlier in its design cycle, and had more room for development and the lessons learned led to greatly improved models in World War II. The Russians failed to keep up and despite newer models coming into service, I-16s remaining the most common Soviet front-line fighter into 1942 despite being outclassed by the improved Bf 109s in World War II. For their part, the Italians developed several monoplanes such as the Fiat G.50 Freccia, but being short on funds, were forced to continue operating obsolete Fiat CR.42 Falco biplanes.
From the early 1930s the Japanese were at war against both the Chinese Nationalists and the Russians in China, and used the experience to improve both training and aircraft, replacing biplanes with modern cantilever monoplanes and creating a cadre of exceptional pilots.  In the United Kingdom, at the behest of Neville Chamberlain (more famous for his 'peace in our time' speech), the entire British aviation industry was retooled, allowing it to change quickly from fabric covered metal framed biplanes to cantilever stressed skin monoplanes in time for the war with Germany, a process that France attempted to emulate, but too late to counter the German invasion.  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.