Sir John Garden set to work in 1934 to design a tank to meet a General Staff specification for a successor to the Vickers mediums, but with a slightly different role to fulfil. The difficulty with the tank specifications of the 1930s was that nobody had any clear idea what they wanted the vehicles to do in the next war. The old ideas of crossing trenches had not entirely died out, yet it was realised that tanks would be needed to act on their own, much in the way that cavalry had done, and also there was a need for armoured recon- naissance. The result of this somewhat baffled thinking was to stipulate a family of three types: cruisers, which were meant to be the cavalry type of machine, yet able to fight it out with other tanks if called upon to do so; infantry tanks which moved at slow speeds with the assaulting infantry, and only had to knock out machine-gun nests (a throwback to 1918); and light tanks for the reconnaissance role. Nobody thought out the armament requirement to cope with these different tasks, and the cruisers were particularly badly served since they were given either the 3 pounder, which was feeble, or the later 2 pounder, which had good armour penetration for its day, but could not fire HE shells. All medium tanks were well supplied with machine-guns, which were quite useless against other armoured vehicles.

With these crippling restrictions around him Sir John Garden produced the first A9 early in 1936. It epitomised all that had served to restrict the design. It was lighter than the mediums so that it could be powered by a commercial engine. At the same time it tried to incorporate all the best features of the Medium Mk.III, and to a great extent succeeded, but only by making everything so much lighter that the armour protection was largely negated. The overall weight was only two-thirds that of the Medium Mk III, and the design weight was even less than this. The general layout was reasonable for its day, with a central turret, engine at the rear and acceptable cross-country performance from the suspension. One of the features which spoiled the A9 was the vertical armour, all of it too thin, and the multitude of angles and corners in which armour-piercing shot could lodge, instead of being glanced off.

A point in the A9's favour, however, was the fact that it was the first British tank to have power (hydraulic) traverse for the turret. This was a substantial step forward, and was to be followed on all succeeding designs. Another notable first was the carriage of an auxiliary engine for starting, battery charging, and driving a fan for the fighting compartment. These were sensible innovations, and went some way to offsetting the failings of the A9 as a fighting tank. The crew was a generous allowance of six men, split into a commander, gunner, loader, driver and two hull machine- gunners. The driving and fighting compartments were combined into one, hence the need for a fan to clear the fumes from three machine-guns and a 3-pounder. The two hull machine-guns were mounted in small sub-turrets in front, one on each side of the driver. The gunners were cramped, and so was the driver, and the whole concept was strongly reminiscent of World War I. The arcs of fire of the machine-guns were limited, and their use was therefore doubtful.

The engine was originally meant to be the Rolls-Royce car engine from the Phantom series. The pilot model, however, showed that the vehicle was under-powered and an AEC bus engine was substituted. This just managed to give the tank a speed of 25mph (40km/h) on the road, but had to be geared down considerably to do it. The suspension could manage the cross- country speed of 15mph (24km/h) but the pilot model at first shed its tracks at these speeds. Trials started in 1936 and at the same time the War Office was changing its policy on tanks generally.

The A9 had begun as a medium tank replacement, but now the cruiser idea was born, and the vehicle became the Cruiser Tank Mk I. The first contract for a limited number was placed in August 1937 with Vickers, which was to build 50. Another contract with Harland and Wolff of Belfast specified a further 75, and these constituted the total production. The limitations of the design were soon obvious and the A13 was put in hand as the next model. The intrinsic limitations of the 2pounder meant that tanks could not deal with strongpoints or pillboxes, and this brought about the concept of the Close Support tank. CS tanks carried large-calibre guns for firing HE and other types of ammunition, and a few CS models of the A9, mounting a short-barrelled 3.7 inch howitzer, were built. The three machine- guns remained. The suspension was a Vickers refinement of the popular multi-bogie system, and it was successful enough to be incorporated into the later Valentine almost without alteration. The steering brakes were mounted externally on the rear sprockets, where they cooled easily, but were perhaps a little exposed to damage. The tracks were narrow, and none too strong, but the low power output of the engine and the relatively gentle gearbox gave them a reasonably long life.

A9s were issued to 1st Armoured Division, which took them to France in 1939 and 1940, and left practically all of them at Dunkirk. The 2nd and 7th Armoured Divisions took the type to Egypt and used it until 1941, by when it was clearly well out of date and out-gunned.