The gun QF 3 inch 20 cwt is the first anti-aircraft gun designed for such purpose by the British. It equipped a large majority of vessels in the Royal Navy during the First World War.

QF 3-INCH 20 CWT - Use

The 3-inch gun FQ 20 cwt Mk I was tested in autumn 1911 as the anti-aircraft gun for the army and navy. Although it is not considered to be satisfactory, it is bought in large quantities.

The Mk II and Mk IIA versions and arm all battleships of the Royal Navy (except for the Audacious (in) ), and all battle cruisers, cruisers scouts (except Pathfinder (in) ), most monitors, some minesweepers and the aircraft carrier HMS Hermes and Campania (in). In all, 395 guns are produced, plus an experimental unit. 85 of these guns were allocated to the air defense of Great Britain, and the army received 48 between April 1917 and April 1918.

The Mk III and Mk IV versions differ from the Mk I by the cylinder head design, making it easier to produce. 44 Mk III were delivered to the Royal Navy and 465 in the British Army between May 1916 and November 1918. 142 Mk IV are delivered to the navy, 131 before the end of the war. In it, only the HMS Bristol has one, and after the conflict ended, the light cruisers of class C and some monitors benefit.