Anti-aircraft warfare
Height finders were also increasing in size; in Britain, the seven-foot optical base World War I Barr & Stroud UB 2 stereoscopic rangefinder was replaced by the nine-foot optical base UB 7  and the eighteen-foot optical base UB 10 (only used on static AA sites). Goertz in Germany and Levallois in France produced five m (16 ft) instruments. However, in most countries the main effort in HAA guns until the mid-1930s was improving existing ones, although various new designs were on drawing boards.
From the early 1930s eight countries developed radar; these developments were sufficiently advanced by the late 1930s for development work on sound-locating acoustic devices to be generally halted, although equipment was retained. Furthermore, in Britain the volunteer Observer Corps formed in 1925 provided a network of observation posts to report hostile aircraft flying over Britain. Initially radar was used for airspace surveillance to detect approaching hostile aircraft. However, the German Würzburg radar put into use in 1940 was capable of providing data suitable for controlling AA guns, and the British  Radar, Gun Laying, Mark I, was designed to be used on AA gun positions and was in use by 1939.
The Treaty of Versailles prevented Germany having AA weapons, and for example, the Krupps designers joined Bofors in Sweden. Some World War I guns were retained and some covert AA training started in the late 1920s. Germany introduced the 8.8 cm FlaK 18 in 1933, the 36 and 37 models followed with various improvements, but ballistic performance was unchanged. In the late 1930s the 10.5 cm FlaK 38 appeared, soon followed by the 39; this was designed primarily for static sites but had a mobile mounting, and the unit had 220 V 24 kW generators. In 1938 design started on the 12.8 cm FlaK.
Britain had successfully tested a new 3.6-inch gun, in 1918. In 1928 a 3.7-inch (94 mm) gun became the preferred solution, but it took six years to gain funding. Production of the QF 3.7-inch gun  began in 1937; this gun was used on mobile carriages with the field army and transportable guns on fixed mountings for static positions. At the same time the Royal Navy adopted a new 4.5-inch (113 mm) gun in a twin turret, which the army adopted in simplified single-gun mountings for static positions, mostly around ports where naval ammunition was available. The performance of the new guns was limited by their standard fuse No 199, with a 30-second running time, although a new mechanical time fuse giving 43 seconds was nearing readiness. In 1939 a machine fuse setter was introduced to eliminate manual fuse setting.
The US ended World War I with two 3-inch AA guns and improvements were developed throughout the inter-war period. However, in 1924 work started on a new 105 mm static mounting AA gun, but only a few were produced by the mid-1930s because by this time work had started on the 90 mm AA gun, with mobile carriages and static mountings able to engage air, sea and ground targets. The M1 version was approved in 1940. During the 1920s there was some work on a 4.7-inch which lapsed, but revived in 1937, leading to a new gun in 1944.
While HAA and its associated target acquisition and fire control was the primary focus of AA efforts, low-level close-range targets remained and by the mid-1930s were becoming an issue.
Until this time the British, at RAF insistence, continued their use of World War I machine guns, and introduced twin MG mountings for AAAD. The army was forbidden from considering anything larger than .50-inch. However, in 1935 their trials showed that the minimum effective round was an impact-fused 2 lb HE shell. The following year they decided to adopt the Bofors 40 mm and a twin barrel Vickers 2-pdr (40 mm) on a modified naval mount. The air-cooled Bofors was vastly superior for land use, being much lighter than the water-cooled "pom-pom", and UK production of the Bofors 40 mm was licensed. The Predictor AA No 3, as the Kerrison Predictor was officially known, was introduced with it.
The 40 mm Bofors had become available in 1931. In the late 1920s the Swedish Navy had ordered the development of a 40 mm naval anti-aircraft gun from the Bofors company. It was light, rapid-firing and reliable, and a mobile version on a four-wheel carriage was soon developed. Known simply as the 40 mm, it was adopted by some 17 different nations just before World War II and is still in use today in some applications such as on coastguard frigates.
Rheinmetall in Germany developed an automatic 20 mm in the 1920s and Oerlikon in Switzerland had acquired the patent to an automatic 20 mm gun designed in Germany during World War I. Germany introduced the rapid-fire 2 cm FlaK 30 and later in the decade it was redesigned by Mauser-Werke and became the 2 cm FlaK 38. Nevertheless, while 20 mm was better than a machine gun and mounted on a very small trailer made it easy to move, its effectiveness was limited. Germany therefore added a 3.7 cm. The first, the 3.7 cm FlaK 18 developed by Rheinmetall in the early 1930s, was basically an enlarged 2 cm FlaK 30. It was introduced in 1935 and production stopped the following year. A redesigned gun 3.7 cm FlaK 36 entered service in 1938, it too had a two-wheel carriage. However, by the mid-1930s the Luftwaffe realised that there was still a coverage gap between 3.7 cm and 8.8 cm guns. They started development of a 5 cm gun on a four-wheel carriage.
After World War I the US Army started developing a dual-role (AA/ground) automatic 37 mm cannon, designed by John M. Browning. It was standardised in 1927 as the T9 AA cannon, but trials quickly revealed that it was worthless in the ground role. However, while the shell was a bit light (well under 2 lbs) it had a good effective ceiling and fired 125 rounds per minute; an AA carriage was developed and it entered service in 1939 as the 37 mm gun M1. It proved prone to jamming, and was eventually replaced in AA units by the Bofors 40 mm. The Bofors had attracted attention from the US Navy, but none were acquired before 1939. Also, in 1931 the US Army worked on a mobile anti-aircraft machine mount on the back of a heavy truck having four .30 calibre water-cooled machine guns and an optical director. It proved unsuccessful and was abandoned.
The USSR introduced a new 76 mm M1931 in 1937, an 85 mm M1938 and developed the 37 mm M1939 (61-K), which appears to have been copied from the Bofors 40 mm. A Bofors 25 mm, essentially a scaled down 40 mm, was also copied as the 25 mm M1939.
During the 1930s solid-fuel rockets were under development in the Soviet Union and Britain. In Britain the interest was for anti-aircraft fire, it quickly became clear that guidance would be required for precision. However, rockets, or "unrotated projectiles" as they were called, could be used for anti-aircraft barrages. A two-inch rocket using HE or wire obstacle warheads - the Z battery - was introduced first to deal with low-level or dive bombing attacks on smaller targets such as airfields. The three-inch was in development at the end of the inter-war period.

Naval aspects: WWI had been a war in which air warfare blossomed, but had not matured to the point of being a real threat to naval forces.  The prevailing assumption was that a few relatively small caliber naval guns could manage to keep enemy aircraft beyond a range where harm might be expected. In 1939 radio controlled target drones became available to the US Navy in quantity allowing a more realistic testing of existing anti-aircraft suites against actual flying and manoeuvring targets. The results were sobering to an unexpected degree.
The United States was still emerging from the effects of the Great Depression and funds for the military had been sparse to the degree that 50% of shells used were still powder fused. The US Navy found that a significant portion of its shells were duds or low order detonations (incomplete detonation of the explosive contained by the shell). Virtually every major country involved in combat in World War II invested in aircraft development. The cost of aircraft research and development was small and the results could be large. So rapid was the performance leaps of evolving aircraft that the British High Angle Control System (HACS) was obsolete and designing a successor very difficult for the British establishment. Electronics would prove to be an enabler for effective anti-aircraft systems and both the US and UK had a growing electronics industry.