At a meeting of the General Staff, an official specification was determined, which included armour, a 2-pounder gun and a road speed of . A subsequent review of the specification by Martel and Percy Hobart approved all round provided cross-country speed could be kept at . Pending the delivery of the A13, an interim design was approved from the A7, A9 and A10, the A9 being selected. Orders for the resulting Mk. I's were limited pending the arrival of the A13.