Development started in October 1942, due to a General Staff order for a successor to the 17-pdr. Basic requirements involved the new gun to be "at least 25% more effective" than the 17-pdr. Initial focus was on development of a 55-pdr gun, however advancements in 17-pdr ammunition meant the benefits achieved by the 55-pdr were no longer as valid, and with considerable disadvantages such as handling and stowage of ammunition. Other options included a 76 mm 17-pdr firing Armour Piercing Composite Rigid shot weighing with a muzzle velocity of a 3"/2.25 Littlejohn firing a Armour-Piercing, Composite Non-Rigid shot at 4,500 ft/s and a 30-pdr gun of calibre. Eventually, a 30pdr, 3.7-inch gun was proposed for a standard calibre, as it had a better margin for improvement. The QF 3.7-inch AA gun was selected as the basis for the 30-pdr anti-tank gun in September 1943 and for pilots to be built. During development, it was proposed that a 37 lb shot would give better performance, and this new design was trialled as the QF 37 pdr EX1 in June 1944. Following firing trials, the 37 lb shot was dropped for a 32 lb shot, and the gun was accepted as the QF 32-pdr. At least one 32-pdr was developed as an anti-tank gun on a wheeled carriage and a single example using a novel form of muzzle brake was installed in a de Havilland Mosquito FB Mk. XVIII as an up-gunned version of the 'Tsetse', in place of the 57 mmMolins gun. While flight trials did not take place until after the war had ended, the aircraft flew and the gun fired without problems. The 32-pdr fired a 32 lb armour-piercing shot at a muzzle velocity of 2,880 ft/s and a Armour-piercing discarding sabot shot at a muzzle velocity of. During firing trials on 28 June 1945, the 32-pdr Shot Mk.3 APDS shot could penetrate RHA at 50° - a line of sight equivalent of at the velocity of 1,487 m/s on impact, which meant that its penetration was more than 17-pdr and 20-pdr APDS rounds, and could even rival early Armour-piercing fin-stabilized discarding sabot rounds in raw penetration. The front hull of the Tiger II and Jagdtiger could be penetrated easily by the 32-pdr gun, and it would have been the only anti-tank gun that could do so in the Second World War. By comparison the M308 HVAP shell could only penetrate at 50-degrees at 100 yards. Development was halted with the end of the war, and the 17-pdr was replaced with the 20-pdr for tank use, the 120mm BAT recoilless rifle for anti-tank use, and the intention to replace it with Anti-tank guided missiles for infantry-use.
