A number of models were produced during the war years, but all worked on the principle of a pneumatic mortar, using compressed air or high pressure steam to fire an explosive projectile at enemy aircraft.

[1] Intended primarily as a stop-gap defensive weapon for British merchant ships, which had been suffering heavy losses from Luftwaffe aircraft flying anti-shipping missions, the low altitude at which such strikes often took place (such as during torpedo attacks by Heinkel He 111s or skip-bombing attacks by Focke-Wulf Fw 200 Condor) meant that a weapon of such limited range and velocity could throw up an effective screen of fire over a vessel, even if only to create a distracting or deterrent effect, obliging the enemy to bomb from greater heights which reduced bombing accuracy.

While ineffective against normal bombing attacks from higher altitudes, the weapon was far cheaper, easier to build and install in great numbers than conventional anti-aircraft artillery.

More appealing yet to the armed forces was the fact that the weapon could be produced using only cast iron and mild steel, both of which were in fairly ready supply at this stage of the war.

[3] The Mk II Projector was developed after a request from the Royal Navy for a version that could be fired using steam in place of compressed air, since the steam-engined trawlers (both fishing and minesweeping) had the former in plentiful supply.

The new version was fitted to a wide variety of ships, from destroyers to minesweepers and motor gun boats: to demonstrate the weapon's versatility, a trial was arranged in Aldershot, Hampshire before Prime Minister Winston Churchill.

(This was prophetic: while possibly apocryphal stories of projectors mounted on trawlers being used to fire 'spuds' (potatoes) at low-flying German aircraft for the want of Mills bombs led to the nickname 'potato thrower'.

[citation needed] Within the Admiralty, the perception was that the Projector was a useful stop-gap weapon in the early years of the war, when other more effective anti aircraft weaponry, such as the Oerlikon 20 mm cannon, were in short supply.