It is uncertain exactly by whom or when explosive devices were first dropped from airplanes. Certainly, however, the concept of the bomber aircraft predated the rise of fighter aircraft by several years. Before the outbreak of World War I in 1914, the French, Germans, Russians, and Austro-Hungarians were developing aircraft specifically designed to carry and release bombs on a target. Great Britain also experimented with the possibility of dropping bombs from aircraft before the war but did not start building aircraft specifically for the task until after the beginning of hostilities. In addition, the Central Powers built a fearsome bombing force around Zeppelin airships before 1914 and used them extensively early in the war.

Combatants used virtually all types of aircraft, including observation and fighter planes, for bombing operations at some time during the war. The British De Havilland 6, for example, could carry either an observer or bombs, but not both. The technological choice, however, was to develop large aircraft that could penetrate enemy defences, defend themselves from aerial attack, and deliver massive amounts of bombs on a target far behind the battle front.

The first genuine bomber to be used in combat was the French "Voisin" airplane, which bombed the Zeppelin hangers at Metz-Frascaty on August 14, 1914. A pusher biplane, the Voisin was rugged and weather-worthy because of its steel airframe. Throughout the war it incorporated increasingly more powerful engines, moving successively up from engines that generated 70 horsepower (52 kilowatts) to those generating 155 horsepower (116 kilowatts). Its bomb-carrying capacity grew from about 132 pounds (60 kilograms) to 661 pounds (300 kilograms) by late in the war. The later Voisins also incorporated a 37mm cannon. The French Aviation Militaire began reorganizing its Voisins into bombardment squadrons in September 1914, which eventually numbered more than 600 aircraft. France used its Voison force after May 1915 to conduct a sustained bombing campaign against the Western Front.

The French efforts were quickly followed by the Imperial Russian Air Service equipped with Igor I. Sikorsky's huge aircraft, the Ilya Mourometz. The world's first four-engine airplane, the Ilya Mourometz first flew on May 13, 1913. Its four engines each generated from 100 to 220 horsepower (75 to 164 kilowatts), its crew of five had sleeping compartments in the rear fuselage, and either three or four machine guns protected it from air attack by. The most advanced Ilya Mourometz could remain aloft for five hours at an altitude of about 9,000 feet (2,743 meters) with a speed of 85 miles per hour (137 kilometres per hour). It could carry 992 and 1,543 pounds (450 and 700 kilograms) of bombs depending on other operational factors. It also enjoyed a 60 percent bombs-on-target rating because of its precise bombsights and excellent training of bombardiers.

Russian Major-General M.V. Shidlovski, commanding the Eskadra VozdushnykhKorablei (Squadron of Flying Ships), equipped his unit with the rugged Ilya Mourometz. Formed specifically to exploit the weakness from the air of the Central Powers on the Eastern Front, Shidlovski made his squadron into a self-contained force. He first used it in combat on February 15, 1915, when it left from its base at Jablonna, Poland, and raided a German base in East Prussia. Between that time and the November 1917 Russian Revolution, Shidlovski's unit made more than 400 bombing raids over Germany and the Baltic states.

The combatants all had different views on bombing strategy. The English, first through the Royal Naval Air Service and then the Royal Air Force, emphasized tactical and revenge bombing. The French were extremely reluctant to bomb behind the German lines, since the range of its bombers did not reach Germany and they did not want to drop bombs on German-occupied France. Also, unoccupied French cities and other targets were close to the front, and retribution bombing would have been easy. 

All countries used bombers in a tactical capacity since bombers could reach areas that ground artillery could not. When an offensive was being mounted, traffic increased in the back lines. Bombers could target these high-traffic areas. During the build-up to the Battle of Mesines Ridge in the summer of 1917, the Germans struck the British munitions supply train. British artillery had to stop firing after three hours when they ran out of ammunition.

 
SPAD XIIIs at Lisle-en-Barrois, with four bombs each hanging beneath their fuselages. October 1918

Industrial bombing targeted factories and mines that were supporting the war effort. The theory was that in destroying the sources of new weapons, the war’s progress could be slowed for a while. And some industrial bombing was simply motivated by revenge. In April of 1915, the Germans dropped chlorine gas on the Allied trenches. French intelligence linked the gas to a factory in Ludwigshafen, and bombers were dispatched to destroy the factory.

 
Strategic bombing had its beginning during World War I when German Zeppelins began raiding London. Small attacks against England were carried out early in the war, but by October 1915, "squadron-size" raids by numerous Zeppelins had begun, always at night and in the dark of the moon.

It was the Germans who first grasped the psychological implications of bombing a civilian population. Using mostly zeppelins in the early years, they instilled fear and panic in the people by flying over their cities. This became a regular practice and made the Germans seem much more powerful and omnipresent in the minds of their enemies.

The bombing of cities remained a moral issue throughout the war. But no one ever believed that cities were off limits for bombing; they had too many industrial sites and government offices that were potential targets. At times, the innocent would be hit by mistake. But the lure of military targets in cities, combined with the psychological power gained by urban bombings meant that they were inevitable. Nothing could make a government and an army look more helpless than to have enemy aircraft descending upon its capital buildings or castles.

On May 23, 1915, Italy entered World War I on the side of the Allies, and while overall poorly prepared for war, it had a competent bombing force. A family of Caproni bombers, almost all of which were trimotors, were first adopted by the Italian military just before the war. The Ca 2, which had the range and reliability to cross the Alps and attack Austro-Hungary, flew the war’s first Italian bombing mission on August 20, 1915. The Ca 2s served as Italy's principal daylight bomber until the appearance of the Ca 3 in 1917. The Caproni Ca 5 series of bombers were the most advanced produced in Italy during the war. This biplane had one pusher and two tractor engines, each generating 200 to 300 horsepower (149 to 224 kilowatts) and capable of carrying a bomb load of 1,190 pounds (540 kilograms) for four hours at 94 miles per hour (152 kilometres per hour). It was susceptible to air attack, however, since it carried only two defensive machines guns. Used throughout the war, Caproni bombers had exceptional range and moderate bomb carrying capabilities. Many were converted to cargo and passenger operations after the war.

The British also building an effective bomber force early in the war. In December 1914, Commodore Murray F. Sueter of the British Admiralty's Air Department. He ordered the development of a "bloody paralyzer of an aeroplane" to bomb Germany. He asked for a two-seat, twin-engine aircraft with a speed of at least 75 miles per hour (121 kilometres per hour) and a carrying capacity of at least six 112-pound (51-kilomgram) bombs. The result was the Handley Page O/100, which went into service with the Royal Navy Air Service in November 1916 and was used at first for daylight sea patrols near Flanders. The plane could carry 16 112-pound (51-kilogram) bombs; used a crew of four; mounted machine guns in the nose, dorsal, and downward from the lower fuselage; and could have its wings folded to fit into standard hangars. Beginning in March 1917, however, they began to be concentrated for night bombing of German U-boat bases, railway stations, and industrial sites. It served effectively until the end of the war.

It was the airplane that the Germans developed in the autumn of 1916, however, that emerged as the most infamous bomber of World War I. The Germans longed to carry out a bombing campaign against England and worked to develop an airplane that could do it after the limitations of the zeppelin became apparent. Their solution: the Gotha G.V. bomber, had two Mercedes engines and a wingspan of over 77 feet (23 meters). It was strong enough to carry more than 1000 pounds (454 kilograms) of bombs and also had a firing tunnel--a hole through the bottom of the airplane that allowed the rear gunner to shoot the enemy below, a common blind spot.

The Gotha bomber was produced in the autumn of 1916 when the limitations of the Zeppelin as a raider had become obvious.  The German High Command ordered that 30 Gotha bombers were to be ready for a daylight raid on London on February 1, 1917, but the machines were not ready until May. The first daylight raid on London was carried out by 14 Gothas on June 13, 1917. On July 7, 22 Gothas raided London. Night raids began in August of 1917, and continued until May 1918, when they were abandoned because of the increasingly heavy losses.  

On May 23, 1917, a fleet of 21 Gothas appeared over the English coastal town of Folkestone. On the deadliest day of bombing yet, 95 people were killed, and England began to panic. At noon on June 13, another Gotha fleet dropped bombs onto London. For the next month, the daily raids on the capital city met with little opposition from the Royal Air Force, angering the population of London. Production levels within the city dropped. Citizens felt that their government was incapable of protecting them. They demanded that the military protect them and stop the bombs. They felt exposed and helpless, just as German military strategists had hoped they would.

Unfortunately for the Germans, the effect of the bombing was not a public uprising against Parliament but a strengthening of the Royal Air Force. In July, the large unwieldy Gothas were forced to resort to night raids so the darkness could shield them from Britain’s Sopwith Camels, light, manoeuvrable planes. By the war’s end, the raids had stopped entirely since the hits were not worth the German aircraft losses. In total, there were 27 Gotha raids. The English reported 835 killed and 1,990 wounded. Damage from the raids totalled £3,000,000, but the loss of production time from workers having to seek shelter in the middle of the day, or suffering exhaustion from having to leave their beds to seek shelter at night, had a far greater impact.