During World War I, anti-aircraft guns and anti-aircraft gunnery developed rapidly. The British Army eventually adopted the QF 3-inch AA gun as the most commonly used type. Shortly before the end of the war a new QF 3.6 inch gun was accepted for service but the end of the war meant it did not enter production. After the war, all anti-aircraft guns except the three-inch gun were scrapped.
However, the war had shown the possibilities and potential for air attack and lessons had been learned. The British had used AA guns in most theatres in daylight, as well as against night attacks at home. They had also formed an AA Experimental Section during the war and accumulated much data that was subjected to extensive analysis. After an immediate post war hiatus, the army re-established peacetime anti-aircraft units in 1922. In 1925 the RAF established a new command, Air Defence of Great Britain, and the Royal Artillery's anti-aircraft units were placed under its command.
In 1924–5 the war office published the two-volume Textbook of Anti-Aircraft Gunnery. It included five key recommendations for heavy anti-aircraft (HAA) guns:
- Shells of improved ballistic shape with HE fillings and mechanical time fuzes
- Higher rates of fire assisted by automation
- Height finding by long-base optical rangefinders
- Centralised control of fire on each gun position, directed by tachymetric instruments which incorporated the facility to apply corrections of the moment for meteorological and wear factors
- More accurate sound-location for the direction of searchlights and to provide plots for barrage fire
Two assumptions underpinned the British approach to HAA fire. First, aimed fire was the primary method and this was enabled by predicting gun data from visually tracking the target with continuous height and range input. Second, that the target would maintain a steady course, speed and height. Heavy anti-aircraft units were to engage targets up to 24,000 feet (7,300 m). Mechanical, as opposed to igniferous, time fuzes were required because the speed of powder burning varied with height so fuze length was not a simple function of time of flight. Automated fire ensured a constant rate of fire that made it easier to predict where each shell should be individually aimed.
During the 1920s Vickers developed the Vickers Predictor, an electro-mechanical computer that took height and range data from an optical rangefinder, applied corrections for non-standard conditions and was used by its operators to visually track a target, its output predicted firing data and fuse setting via the "mag-slip" electrical induction system to dials on each gun in a battery, the gun layers moved the gun to match pointers on the dials. The three-inch AA guns were modified accordingly.
In 1928 the general characteristics for a new HAA gun were agreed on; a bore of 3.7 inches (94 mm) firing 25 pounds (11 kg) shells with a ceiling of 28,000 feet (8,500 m). However, finances were very tight and no action was taken until the 1930s, when the specification was enhanced to a 28 pounds (13 kg) shell, 3,000 feet per second (910 m/s) muzzle velocity, a 35,000 feet (11,000 m) ceiling, a towed road speed of 25 miles per hour (40 km/h), maximum weight of eight tons and an into action time of 15 minutes.
In 1934 Vickers Armstrong produced a mock-up and proceeded to develop prototypes of the weapon, which was selected and passed acceptance tests in 1936. However, the weight specification was exceeded and the muzzle velocity not achieved. Furthermore, the initial mechanical time fuze, No. 206, was still some years from production so the igniferous No. 199 had to be used, and its lesser running time limited the effective ceiling. Gun production started the following year.
On 1 January 1938, the British air defences had only 180 anti-aircraft guns larger than 50 mm, and most of these were the older three-inch guns. This number increased to 341 by the September 1938 (Munich Crisis), to 540 in September 1939 (declaration of war), and to 1,140 during the Battle of Britain. Production continued until 1945, averaging 228 guns per month throughout the period. Guns were also manufactured in Australia.
Being a high velocity gun, with a single charge and firing substantial quantities of ammunition, meant that barrel life could be short. By the end of 1940 the barrel situation was becoming critical. Some of the substantial numbers of spare barrels required were produced in Canada.
In British service the gun replaced the three-inch AA gun in HAA batteries of the Royal Artillery, almost always in HAA regiments, which were usually in an anti-aircraft brigade. Each regiment usually had three batteries, each of eight guns in two troops. A total of 212 of these HAA regiments, Royal Artillery, and two each of the Royal Marines and Royal Malta Artillery were eventually formed. Other World War II users were India (about 14 regiments), Canada (two or three regiments), East and West Africa (five regiments), and Australia (equivalent of about 13 regiments).