From the early twenties until the mid- to late thirties, the standard fare of the Royal Tank Corps was the Vickers Medium Mk II supported by various marks of light tank. The Medium Mk ll's projected replacement - logically enough to be known as the Medium Mk III - fell victim to the financial cutbacks of the thirties and did not get beyond the prototype stage. Sir John Carden of Vickers-Armstrong, the company behind the Medium Mk III, then designed a cheaper alternative designated the A9, which incorporated some of the improvements of the Medium Mk III but was lighter and powered by an existing, readily available engine. First choice was to have been the 7.67-litre Rolls-Royce Phantom II, but that proved not to have the required power, so AEC's 9.64-litre A179 bus engine was selected instead. Two different types of main armament were initially proposed, a three-pounder and a3.7in howitzer. Trials commenced in July 1936 and production started in 1937, by which time the proposed three-pounder had been supplanted by a two-pounder which in the meantime had become the new standard tank gun. Of more long-term importance was the War Office's decision to dispense with the old medium class of tanks and to instead divide requirements into three classes, light, cruiser and infantry. Accordingly, the A9, which would have preciously fallen into the medium class, became the Cruiser, Tank, Mk I (A9) or Mk I CS (A9) in the case of the howitzer-armed close-support variant.
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