Operation Overlord, the largest landing in all of WWII, opened the Western Front. A significant role in this operation was played by Cromwell tanks.
In the end of 1940, the British Staff announced a tender for the creation of a heavy cruiser tank. According to the requirements, the vehicle would weigh around 25 tons, have 70 mm of armour and a 57 mm gun.
Similar tanks were already in possession of the British army, the pre-war Covenanter and the Crusader, but according to pre-war requirements, they were only armed with 40 mm guns.
The British needed a completely new and more powerful cruiser tank. In January of 1941, a special commission investigated three projects of what would later be named "Cromwell". Production was scheduled for spring of 1942. The commission's final choice was the A24 project, which relied on existing components and assemblies.
The first Cromwells were not so well designed. Old engines turned out to be completely unusable for them. These tanks, renamed "Cavalier", were repurposed for training.
Around the time that this new tank was being tested, British command changed the requirements for tank armament. Experience in North Africa showed that a tank gun should not only be used to destroy enemy tanks, but also to destroy enemy infantry and fortifications. In orther words, there was a need to attack less protected targets with explosive shells. Sherman and Grant tanks had this ability, and British engineers were ordered to create a similar 75 mm gun. This delayed the appearance of combat-ready Cromwells by another two years.
By May of 1944, engineers resolved most defects, and a month later Cromwell tanks (still with the old 57 mm gun) first drove into battle with the British-American forces.
Original article available here.