The QF 3.7-inch AA was Britain’s primary heavy anti-aircraft gun during World War II. It was roughly the equivalent of the German Flak 8.8 cm and American 90 mm, but with a slightly larger calibre of 3.7 inches, approximately 94 mm. Production began in 1937 and it was used throughout World War II in all theatres except the Eastern Front. It remained in use after the war until AA guns were replaced by guided missiles beginning in 1957.
The gun was produced in two versions, one mobile and another fixed. The fixed mounting allowed more powerful ammunition, Mk. VI, which gave vastly increased performance. Six variants of the two designs were introduced. The gun was also used as the basis for the Ordnance QF 32-pounder anti-tank gun variant used on the Tortoise heavy assault tank.
The decision to manufacture the 3.7-inch anti-aircraft gun in Australia was made as a result of Imperial Defence Conference in May/June 1937 as a contribution to Empire defence. In October 1937 the Munitions Supply Board sent two of its officers, including Mr V. Parker, the Assistant Manager of the Ordnance Factory, to England to study the manufacture of the 3.7-inch gun, and at the same time the purchase of machine tools and plant was authorised. The first production order for 130 guns was received from the Army on 22 March 1939. Ten months after receiving the drawings the Ordnance Factory turned out its first gun in May 1940. This was the beginning of the factory’s large scale contribution to the stock of weapons used in the war of 1939-1945. In August 1940 four guns were shipped to Darwin. Others were sent to Britain and set up in Hyde Park, London where they played their part in defending the city in the great raids of 1940. Other guns were sent to Singapore, only to be captured by the Japanese.
Production of eight guns per month was achieved by January 1941, increasing to 12 in February 1941, 16 in December 1941, and eventually 22 in June 1942. The guns cost £7,500 ($15,000) each as against £10,000 ($20,000) in Britain. A total of 600 fixed mounting guns were produced.