The Cromwell and the related Centaur were the product of further development of British cruiser tanks, and they were designed as the replacement for the Crusader tank, which although not yet in service would become obsolete in time. In late 1940, the General Staff set out the specifications for the new tank, and designs were submitted in early 1941. The tank would be fitted with the QF 6 pounder gun with the expectation that it would enter service in 1942.
Due to the typical rushed production and lack of components, the A24 Cavalier, then known as "Cromwell I", built by Nuffield had far too many problems to see active combat service. One of the key problems was that its Nuffield-built Liberty engine was simply not up to the task. It had been ordered as it was based on tried equipment and therefore should have entered service with minimal delay.
Leyland and Birmingham Railway Carriage & Wagon had been involved in the development and had offered similar designs to Nuffield. A second specification for a better tank was the General Staff A27. The tank would be fitted with the QF 6 pounder gun with the expectation that it would enter service in 1942. Once it became clear there would be delays, a programme was set in place to fit the 6 pounder to the Crusader to get some 6 pounder tanks in service.
At the same time, a new engine was designed to be a tank powerplant. The Meteor engine was based on the powerful Rolls-Royce Merlin engine used in aircraft such as the Spitfire. Rolls-Royce, Leyland and BRC&W produced a prototype by January 1942 based on the Crusader but using the Meteor. With nearly 600 hp (450 kW) it proved to be exceptionally mobile when trialled. Leyland were lined up to produce the Meteor but withdrew in mid-1941 as they had doubts about being able to provide sufficient cooling. Rolls-Royce, the makers of the Merlin, were already fully committed to manufacturing the Merlin and could not spare the facilities for the Meteor, and so manufacture was passed to the Rover Car Company.
The General staff issued new specifications to cover the tanks. The BRC&W design using the Meteor was A27M (or "Cromwell III") and Leyland's version of it to take the Liberty was A27L ("Cromwell II"). Nuffields A24 with the Liberty was the Cromwell II. The naming was reworked in November 1942 with the A27L as Centaur, A27M as Cromwell and A24 as Cavalier.
Production began in November 1942. It would take considerable time for Rover to make ready production lines for the Meteor, and it was not until a few months later, in January 1943, that sufficient Meteor engines were available and the A27M Cromwell began production. The Centaur production design allowed for the later conversion to the Meteor engine and many Centaurs would be converted to Cromwells before use.
The frame was of rivetted construction though welding was used later. The armour plate was then bolted to the frame; large bosses on the outside of the plate were used on the turret. Several British firms besides Leyland contributed to production of the Cromwell and Centaur including LMS Railway, Morris Motors, Metro-Cammell, Birmingham Railway Carriage and Wagon Company and English Electric Some variants were produced with 14-inch-wide (360 mm) tracks, later 15.5-inch tracks were used.
The suspension was of the Christie type with long helical springs (in tension) angled back to keep the hull sides low. Of the five roadwheels each side, four had shock absorbers. The tracks were driven by sprocketed wheels at the rear and tension adjusted at the front idler; this being standard British practice. The side of the hull was made up of two spaced plates, the suspension units between them, and the outer plate having cutouts for the movement of the roadwheel axles. The gearbox had five forward and one reverse gears. The first gear was for "confined spaces, on steep inclines or...sharp turns"
The Meteor engine delivered 540 hp at 2,250 rpm. This was the maximum rpm which was limited by governors built into the magnetos. Fuel consumption on "pool" petrol (67 octane) was between 0.5 and 1.5 miles per gallon depending on terrain.
The driver was sat on the right in the front of the hull, separated from the hull gunner by a bulkhead. The driver had two periscopes and a visor in the hull front. The visor could be opened fully or a small "gate" in it opened; in the latter case a thick glass block protected the driver. A bulkhead with access holes separated the driver and hull gunner from the fighting compartment. A further bulkhead separated the fighting compartment from the engine and transmission bay. The engine compartment drew cooling air in through the top of each side and the roof and exhausted it to the rear. To allow fording through up to 4 ft (1.2 m) deep water a flap could be moved over to cover the lowermost air outlet. Air for the engine could be drawn from the fighting compartment or the exterior; it was then passed through oil bath cleaners.
The Cromwell still had revisions to make before service, most notably changing from the QF 6-pounder (57 mm) to the ROQF 75 mm gun, which was an adaptation of the 6 pounder design to fire the ammunition of the US M3 75 mm gun, which gave it a better HE round to use in infantry support. This meant the 75 would use the same mounting as the 6 pounder however it was not until June 1944 that Cromwell first saw action during Operation Overlord, the Allied invasion of Normandy. It had a mixed reception by crews. It was faster and had a lower profile than the Sherman tank and thicker frontal armor; 3 inches (76mm) versus the 2 inches (51mm) of the Sherman. On later Cromwells this was increased incrementally, first to 3 1/4 inches (82mm), then finally to 4 inches (102mm). The 75 mm gun, though able to fire a useful HE shell, was not as effective against armour as the 6 pdr or the Ordnance QF 17 pounder gun though it was more powerful than the original 75 mm gun mounted on the Sherman. A derivative of Cromwell was begun to take the 17 pounder, this fell behind and in practice the majority of the 17 pounder gun armed tanks to see service in the war were Firefly variant of the Sherman.
There was a 7.92 mm Besa machine gun mounted coaxially to the main armament operated by the gunner. A second was "gimbal" mounted in the front of the hull. The mounting gave 45 degress of coverage to the front (it had 25 degrees of vertical movement as well) and sighting was by a No. 35 telescope which was connected through a linkage to the mounting.
In the top of the turret was mounted a 2 inch "bombthrower" angled to fire forward. Thirty smoke grenades were carried for it.