High Quality Content by WIKIPEDIA articles! The 3.7-Inch QF AA was Britain's primary heavy anti-aircraft gun during World War II, the equivalent of the German 88 mm FlaK with a slightly larger calibre of 94 mm. It remained in use after the war until AA guns were replaced by guided missiles in the late 1950s. Prior to World War II most countries had not seriously considered upgrading their anti-aircraft systems. The rapid advances in aircraft performance in the early 1930s due to the widespread introduction of supercharging, notably a dramatic increase in operating altitude, meant that guns had marginal performance against newer aircraft. Adding to the concern was the much higher speeds of aircraft, which reduced detection times and seemed to make engagements very difficult to arrange. Given these problems, the British relied almost entirely on the Royal Air Force's bomber fleet as a deterrent force, ignoring the defence almost entirely.