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1914–1918: World War I

The word "fighter" was first used to describe a two-seat aircraft carrying a machine gun (mounted on a pedestal) and its operator as well as the pilot. Although the term was coined in the United Kingdom, the first examples were the French Voisin pushers beginning in 1910, and a Voisin III would be the first to shoot down another aircraft, on 5 October 1914.

However at the outbreak of World War I, front-line aircraft were mostly unarmed and used almost exclusively for reconnaissance. On 15 August 1914, Miodrag Tomić encountered an enemy airplane while on a reconnaissance flight over Austria-Hungary which fired at his aircraft with a revolver, so Tomić fired back. It was believed to be the first exchange of fire between aircraft. Within weeks, all Serbian and Austro-Hungarian aircraft were armed.

Another type of military aircraft formed the basis for an effective "fighter" in the modern sense of the word. It was based on small fast aircraft developed before the war for air racing such with the Gordon Bennett Cup and Schneider Trophy. The military scout airplane was not expected to carry serious armament, but rather to rely on speed to "scout" a location, and return quickly to report, making it a flying horse. British scout aircraft, in this sense, included the Sopwith Tabloid and Bristol Scout. The French and the Germans didn't have an equivalent as they used two seaters for reconnaissance, such as the Morane-Saulnier L, but would later modify pre-war racing aircraft into armed single seaters. It was quickly found that these were of little use since the pilot couldn't record what he saw while also flying, while military leaders usually ignored what the pilots reported.

Attempts were made with handheld weapons such as pistols and rifles and even light machine guns, but these were ineffective and cumbersome. The next advance came with the fixed forward-firing machine gun, so that the pilot pointed the entire aircraft at the target and fired the gun, instead of relying on a second gunner. Roland Garros bolted metal deflector plates to the propeller so that it would not shoot itself out of the sky and a number of Morane-Saulnier Ns were modified. The technique proved effective, however the deflected bullets were still highly dangerous.

Soon after the commencement of the war, pilots armed themselves with pistols, carbines, grenades, and an assortment of improvised weapons. Many of these proved ineffective as the pilot had to fly his airplane while attempting to aim a handheld weapon and make a difficult deflection shot. The first step in finding a real solution was to mount the weapon on the aircraft, but the propeller remained a problem since the best direction to shoot is straight ahead. Numerous solutions were tried. A second crew member behind the pilot could aim and fire a swivel-mounted machine gun at enemy airplanes; however, this limited the area of coverage chiefly to the rear hemisphere, and effective coordination of the pilot's maneuvering with the gunner's aiming was difficult. This option was chiefly employed as a defensive measure on two-seater reconnaissance aircraft from 1915 on. Both the SPAD S.A and the Royal Aircraft Factory B.E.9 added a second crewman ahead of the engine in a pod but this was both hazardous to the second crewman and limited performance. The Sopwith L.R.T.Tr. similarly added a pod on the top wing with no better luck.

An alternative was to build a "pusher" scout such as the Airco DH.2, with the propeller mounted behind the pilot. The main drawback was that the high drag of a pusher type's tail structure made it slower than a similar "tractor" aircraft.A better solution for a single seat scout was to mount the machine gun (rifles and pistols having been dispensed with) to fire forwards but outside the propeller arc. Wing guns were tried but the unreliable weapons available required frequent clearing of jammed rounds and misfires and remained impractical until after the war. Mounting the machine gun over the top wing worked well and was used long after the ideal solution was found. The Nieuport 11 of 1916 used this system with considerable success, however, this placement made aiming and reloading difficult but would continue to be used throughout the war as the weapons used were lighter and had a higher rate of fire than synchronized weapons. The British Foster mounting and several French mountings were specifically designed for this kind of application, fitted with either the Hotchkiss or Lewis Machine gun, which due to their design were unsuitable for synchronizing. The need to arm a tractor scout with a forward-firing gun whose bullets passed through the propeller arc was evident even before the outbreak of war and inventors in both France and Germany devised mechanisms that could time the firing of the individual rounds to avoid hitting the propeller blades. Franz Schneider, a Swiss engineer, had patented such a device in Germany in 1913, but his original work was not followed up. French aircraft designer Raymond Saulnier patented a practical device in April 1914, but trials were unsuccessful because of the propensity of the machine gun employed to hang fire due to unreliable ammunition. In December 1914, French aviator Roland Garros asked Saulnier to install his synchronization gear on Garros' Morane-Saulnier Type L parasol monoplane. Unfortunately the gas-operated Hotchkiss machine gun he was provided had an erratic rate of fire and it was impossible to synchronize it with the propeller. As an interim measure, the propeller blades were fitted with metal wedges to protect them from ricochets. Garros' modified monoplane first flew in March 1915 and he began combat operations soon after. Garros scored three victories in three weeks before he himself was downed on 18 April and his airplane, along with its synchronization gear and propeller was captured by the Germans. Meanwhile, the synchronization gear (called the Stangensteuerung in German, for "pushrod control system") devised by the engineers of Anthony Fokker 's firm was the first system to enter service. It would usher in what the British called the " Fokker scourge " and a period of air superiority for the German forces, making the Fokker Eindecker monoplane a feared name over the Western Front, despite its being an adaptation of an obsolete pre-war French Morane-Saulnier racing airplane, with poor flight characteristics and a by now mediocre performance. The first Eindecker victory came on 1 July 1915, when Leutnant Kurt Wintgens, of Feldflieger Abteilung 6 on the Western Front, downed a Morane-Saulnier Type L. His was one of five Fokker M.5 K/MG prototypes for the Eindecker, and was armed with a synchronized aviation version of the Parabellum MG14 machine gun. 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.