The A10 was developed by Sir John Carden of Vickers in 1934 by the adaptation of his A9 design. The A10 specification called for armour of up to 1 inch (25 mm) standard (the A9 was 14 mm (0.55 in)); a speed of 10 mph (16 km/h)) was acceptable. The two sub-turrets present on the A9 were removed, and extra armour bolted onto that already present on the front and sides of the hull, along with all faces of the turret, providing approximately twice the armour in most areas. The A10 was two tonnes heavier than the A9, but used the same 150 bhp engine, and as a consequence the tank's top speed was cut from 25 miles per hour (40 km/h) to 16 miles per hour (26 km/h).
The turret armament consisted of a QF 2-pounder (40-mm) gun and a coaxial .303 Vickers machine gun. For the production version there was a 7.92 mm BESA machine gun mounted in the hull in a barbette to the right of the driver. This was added to give extra firepower but at the expense of simplicity - the Vickers and the BESA using different ammunition. The tank had a total crew of five (Commander, gunner, loader, driver and hull machine gunner), and there was no separation between the driver's compartment and the fighting compartments.
The prototype ("Tank, Experimental A10E1") was completed in 1936, a few months after the A9 prototype. Carden had died in an air crash in 1935 and development was slower than expected. In 1937, the A10 was dropped as an infantry support tank, but in 1938 it was decided to produce it as a "heavy cruiser".
The A10 was accepted for service - initially as "Tank, Cruiser, Heavy Mk I" and then "Tank, Cruiser A10 Mk 1" and finally "Tank, Cruiser Mk II". Production was ordered in July 1938. Total production was 175 vehicles, including the 30 CS versions (see below); 45 were built by Birmingham Railway Carriage and Wagon Company, 45 by Metropolitan-Cammell, 10 by Vickers. In late 1939, another order was placed with Birmingham Railway Carriage and Wagon Company, this time for a larger order of 75 vehicles. - entering service in December 1939, but was something of an oddity - it had been intended to sacrifice speed for armour like an Infantry tank, but was still relatively poorly armoured, and was, as a result, not effective.