After the initial battles in the Western Desert in 1941 and 1942 it was seen that the British didn't have guns that were able to easily defeat the German armored vehicles. The General Staff told the Tank Board that there was a need for a cruiser tank with an antitank gun that could penetrate the armor on German tanks.
In January 1943 official policy changed that medium tanks didn't have to use the 6 pdr but could have the 75 mm instead. The Challenger had its limitations and it was decided to develop a tank that could replace it.
In February/May 1943, Leyland Motors started to design the Comet. It was to be built with as many of the components of the Cromwell as possible.
By July 1943 a proposal was presented to the General Staff and in September 1943 a mock-up was ready for inspection. In October 1943 an order was placed for three prototypes and one hull (that could be fired at). The order was placed with the thought that the A41 Centurion project would soon replace it. But with the delays in the A41 the Comet was eventually put on high priority production status.
The hull and turret were all welded which were part cast and part rolled. There were doors that opened to the side for the driver and hull gunner. The turret was made up of three welded bent armored plates.
The turret was traversed by electrical power from the main engine. Armored storage bins were over the tracks and behind the turret. On the rear of the turret was a storage bin that had heavier items and spare tracks were hung from it. These provided a counter-weight helping to balance the turret.
The driver was on the right and the hull machine gun gunner on the left. The commander and gunner sat on the left of the main armament, with the gunner on the left. They were all in a suspended turret basket. The cupola for the commander was the same as on the Cromwell and provided good all around vision.
The bow plate was sloped, but to install a hull machine gun a vertical section was added. Some crew felt that the hull machine gun wasn't of much value as it wasn't used as much.
Initially the 17 pdr was considered to arm the Comet, but it came down to choosing between the High Velocity (HV) 75 mm or the American 76 mm. Using the 76 mm would have required that the gun would be modified to fit the layout of British turrets.
The High Velocity (HV) 75 mm was a modified 17 pdr (76.2 mm) that had been designed by Vickers-Armstrong. It could fire the same shell as the 17 pdr but the barrel wasn't as long so muzzle velocities was reduced. The performance of the 77 mm was nearly as good as the 17 pdr.
To reduce confusion with other tank weapons this gun was designated the 77 mm.
It could also fire a 20 lb HE shell.
A stronger suspension was needed and return rollers were added.