The 3.7-inch Heavy AA Gun

Developed in the late thirties from a British Army specification for a weapon between the then current 3 and 4.7 inch calibre Anti-Aircraft guns.
Research suggested that a 3.7-inch gun firing a shell of approximately 25lb weight could fill the gap. Thus, in 1933, a specification for a 3.7" gun weighing eight tons was issued. The gun had to be capable of being put into action in 15 minutes and of being towed at 25mph.
A design by Vickers-Armstrong was accepted and the pilot model passed proof in April 1936 with production being authorised a year later. The first production guns were delivered in January 1938. It was extremely advanced and complicated, yet regarded as one of the best of its type.
The first production model could fire a 28lb 94mm shell to 30,000 feet at a rate of 20 rounds per minute. It was crewed by seven men or, from 1941 onwards, increasingly by women when deployed for Home Defence.
Production was initially slow, particularly due to its complicated carriage but modifications to the design to simply it for production helped and peak production was reached for guns in March 1942 (228) and for mountings June 1942 (195).
The guns were made in the United Kingdom until 1943 and then in Canada for the rest of the war. Unlike the Germans, who used their 88mm guns as both anti-aircraft and anti-tank, the QF 3.7 was too heavy to be used in mobile warfare and was damaged by low elevation firings. The last version, Mark VI, was only employed in static positions.

Source: Days of Glory