As the Guy company did not have sufficient production capacity to produce sufficient Guy Armoured Cars as well as other vehicles, so shortly after war broke out the Rootes Group were approached to produce an armoured car - at the time the terminology "Tank, Light (Wheeled)" was used by the Army. Working from the Guy design, Karrier designed a vehicle using as a basis their KT 4 artillery tractor chassis (already in production for the Indian Army) and the armoured body of the Guy Armoured Car. Karrier moved the KT4 engine to the rear and fitted welded bodies and turrets provided by Guy. As it had been based on proven elements, trials of prototypes passed without serious issues and an order for 500 was placed in 1940 and first deliveries made in 1941.