Machine-guns were fitted to the planes, and the aerial duels began in earnest.

The appearance in 1915 of the German Fokker E-2 monoplane, which featured a machine-gun synchronized to shoot between the blades of the spinning propeller, opened the era of air combat proper.

The fight for control of the air over the trenches fuelled rapid technical development. Private companies had been producing aircraft since the early days of aviation, and names like Nieuport, Sopwith, and Fokker became famous, their aircrafts' relative merits tested against one another in combat. Technical advantage shifted back and forth across the lines as new aircraft capable of flying higher and faster or of carrying more weaponry were introduced. By 1918, the skies were contested by superb fighter aircraft such as the German Fokker D.VII, French Spad 13, and British S.E.5 and Sopwith Camel, which operated at speeds of up to 200 km/h (125 mph), and altitudes of 6,100 m (20,000 ft).

The pilots who flew these aircraft became the best-known fighters of the war. The German ace Manfred von Richthofen, known as the "Red Baron", ran up a total of 80 victories before his death on April 21, 1917. Rene Fonck, a French pilot with 75 victories, was the highest-ranking ace to survive the war. Other top fighter pilots of the war were Major Edward "Mick" Mannock (United Kingdom, 73 victories); Major William "Billy" Bishop (Canada, 72 victories); Captain Ernst Udet (Germany, 62 victories); and Captain Edward V. Rickenbacker (United States, 25 victories). While public attention focused on the fighter pilots, other fliers were exploring a variety of military roles for the aeroplane. Throughout the war, observation and artillery-spotting were the most critical tasks performed by aircraft. Light bombers began to attack troops on the ground, although their contribution to the thousands of tonnes of explosive rained down on northern France was small compared to that from the guns of the artillery. Bombs were small, filled with at most 112 kg (250 lb) of explosive; even so, few aircraft could carry more than two.

German Zeppelin airships bombed cities in Belgium, England, and France during the years 1914 to 1917. During the final year of the war, German fliers continued the air attacks on London and other cities with twin-engine Gotha aircraft and giant four-engine bombers like the Zeppelin R-16, with a wingspan of more than 42 m (138 ft) and a payload of 2,040 kg (4,500 lb). However, only six R-16s were used against Britain, and their ungainliness became apparent as landing accidents destroyed two of them. Britain's response was the twin-engined Handley-Page 0-400, and then the four-engine 0-1500, a giant designed to reach Berlin. The night bombing of cities prompted attempts to shoot down the attackers with fighter aircraft.