The Chieftain was a radical evolutionary development of the successful Centurion line of tanks that had emerged at the end of the Second World War. The British had learned during the war that their tanks often lacked sufficient protection and firepower compared to those fielded by the enemy, and that this had led to high casualty levels when faced with the superior German tanks in World War II.
Leyland, who had been involved in Centurion, had built their own prototypes of a new tank design in 1956, and these led to a War Office specification for a new tank. The design was accepted in the early 1960s. Chieftain was designed to be as well protected as possible and to be equipped with a powerful 120 mm rifled gun.
The heavy armour came at the price of reduced mobility, chiefly due to engine power limitations, which was perhaps the Chieftain’s main drawback. The engine selected took the multi-fuel route and as introduced gave less than the planned output; improvements to the engine did not increase power to the desired value.