In 1936, the British War Office designated two different kinds of tanks for future development: heavily armoured infantry tanks to be used in close co-operation with infantry during attacks, and fast mobile cruiser tanks designed to make forays deep into enemy territory.
In 1934, Sir John Carden of Vickers-Armstrong was asked to provide a "reasonably cheap tank" as a replacement for some of the medium models then in use. The pilot model of his design was finished in 1936 and given the designation A9E1.
It incorporated the best features of the earlier Mk III Light Tank, and was powered by a commercial petrol engine. However, this was still in the time of the Great depression and the tank had a number of cost-cutting measures applied. It was the first British tank to have a centrally located turret and to have powered traverse. The system was by Nash & Thompson and similar to that being introduced on the Vickers Wellington bomber aircraft. The armour was light, with a maximum of 14 mm thickness. Many armour faces were vertical, and there were numerous shot traps, but it could achieve 25 mph and carried the new high velocity QF 2-pounder (40 mm) gun.
The driver's compartment and the fighting compartments were not separated. As well as the turret armament, which consisted of an Ordnance QF 2 pounder gun and a coaxial Vickers machine gun, there were two small turrets either side of the driver's compartment, each with a Vickers machine-gun. Both these smaller turrets were permanently manned, which gave the tank a total crew of 6 (Commander, gunner, loader, driver and two machine-gunners).
The A9E1 was tested against other designs and -although lacking in some areas - it was accepted in 1937 as an interim design until a Christie suspension cruiser tank could be delivered. An order was placed for 125. Seventy-five were built by Harland and Wolff, and the other 50 were built by Vickers. Originally, a Rolls-Royce car engine was used, but this proved underpowered and was replaced by an AEC bus engine.
The later Valentine Infantry tank essentially used the same lower hull and suspension, though with considerably more armour.
The A9 weighed 12 tons, was 5.8 metres long, 2.65 metres high, 2.5 metres wide, and had a top speed of 25 mph on road and 15 mph off. Its maximum road range was 150 miles. The ammunition load was 100 2-pounder rounds and a total of 3,000 rounds for the three Vickers machine guns.