The Sherman Firefly was a tank used by the United Kingdom and some armoured formations of other Allies in the Second World War. It was based on the US M4 Sherman, but was fitted with the more powerful 3-inch (76.2 mm) calibre British 17-pounder anti-tank gun as its main weapon. Originally conceived as a stopgap until future British tank designs came into service, the Sherman Firefly became the most common vehicle mounting the 17-pounder in the war. During the war, the British Army made extensive use of Sherman tanks. Though they expected to have their own tank models developed soon, the previously rejected idea of mounting the 17-pounder in the Sherman was eventually accepted, despite initial government resistance. This proved fortunate, as both the Cruiser Mk VIII Challenger and Cruiser Mk VIII Cromwell tank designs experienced difficulties and delays. After the difficult problem of getting such a large gun to fit in the Sherman's turret was solved, the Firefly was put into production in early 1944, in time to equip Field Marshal Bernard Montgomery's 21st Army Group for the Normandy landings. It soon became highly valued, as its gun could almost always penetrate the armour of the Panther and Tiger tanks it faced in Normandy, something no other British Army tank could reliably do at that time. In recognition of this, German tank and anti-tank gun crews were instructed to attack Fireflies first. Because the Firefly had a visibly longer barrel, crews tried to camouflage it so the tank would look like a normal 75 mm-gun Sherman from a distance. Between 2,100 and 2,200 were manufactured before production wound down in 1945. Origins The idea of fitting a 17-pounder gun into a Sherman tank had initially been rejected by the Ministry of Supply's Tank Decision Board. Although the British Army had made extensive use of the American-built Sherman, it was intended that a new generation of British tanks would replace it in the anti-tank role. First, there was the Cromwell, which was expected to use the Vickers high velocity 75 mm gun; this gun would have had superior anti-tank performance to the US 75 mm and 76 mm guns that were mounted in the Sherman. The second was the A30 Challenger, which was based on the Cromwell but with the even more powerful 17-pounder gun. These two tanks—and their successors, the Comet and the Centurion, which were already on the