The QF 3.7-inch AA was Britain's primary heavy anti-aircraft gun during World War II.

They had also formed an AA Experimental Section during the war and accumulated much data that was subjected to extensive analysis.

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.

[5] During the 1920s, Vickers developed the Vickers range clock (Predictor No 1), 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.

[5] In 1928, the general characteristics for a new HAA gun were agreed, a bore of 3.7 inches (94 mm) firing 25 lb (11 kg) shells with a ceiling of 28,000 feet (8,500 m).

Financial stringency led to no action being taken until the 1930s, when the specification was enhanced to a 28 lb (13 kg) shell, 3,000 ft/s (910 m/s) muzzle velocity, a 35,000 feet (11,000 m) ceiling, a towed road speed of 25 mph (40 km/h), maximum weight of eight tons and an into action time of 15 minutes.

In 1934, Vickers-Armstrongs produced a mock-up and proceeded to develop prototypes of the weapon, which was selected over a competitor by the state Design Department and passed acceptance tests in 1936.

The other used a travelling platform (Mounting Mk II) with detachable wheels for guns to be used in static positions but which could be re-positioned.

The carriage included the recoil system, laying arrangements, fuse setting and loading machinery.

The Mk IIC mounting enabled fully automatic engagements, apart from putting shells into the feed to the machine fuze setter.

The introduction of the VT fuse later in the war further increased the gun's effectiveness, and was particularly useful against the V-1 flying bomb.

This smoothed the two driving bands of a new design shell giving reduced air resistance and hence better ballistic performance, and causing far less barrel wear.

The British definition of effective ceiling at the start of World War II was "that height at which a directly approaching target at 400 mph can be engaged for 20 seconds before the gun reaches 70° elevation"[12] From 1943, radar direction of mechanically slaved 3.7" AA batteries was deployed in Kent.

Like other British guns, the 3.7 had a secondary direct fire role for defending its position against tank attack.

Its size and weight - two tons heavier than the German Flak 8.8 cm - made it tactically unsuitable for use in forward areas.

The mounting and recuperating gear were also not designed to handle the strain of prolonged firing at low elevations.

There were few 3.7-equipped heavy anti-aircraft regiments in the field army and most were not subordinate to divisions where the anti-tank capability was required.

The arrival of the smaller 76 mm (3-inch) calibre 17-pdr anti-tank gun finally obviated the need.

With the declining threat from the Luftwaffe in the later stages of the war, under-employed 3.7 units were called upon to supplement the field artillery in both the North West Europe and Italian theatres, where the accuracy and effectiveness of the 3.7 with mechanical Fuse 207 at ranges up to 20,000 yards (18,000 m) and all-round traverse was valued by artillery commanders.

Using the 207 or VT fuse allowed the gunners to deliver precise airbursts above targets such as enemy batteries or mortar positions.

By the time of Operations Veritable and Plunder (the Rhine crossing) in early 1945, HAA regiments were fully integrated into corps-level fire plans.