In the mid-1920s, with train load sizes increasing to and from the South West, the Great Western Railway were faced with the need for even more powerful locomotives, capable of hauling the heavy expresses at an average speed of 60 mph. The main routes were limited to a 20 ton axle weight though, so a number of the Great Western’s routes had to be upgraded, including the strengthening of some bridges, before the Great Western’s General Manager, Sir Felix Pole, would give his Chief Mechanical Engineer, Charles Collett, permission to proceed with the design and construction of a "Super-Castle".

The result was the King class 4-6-0, which emerged from Swindon works in June 1927, a design that increased the length and wheelbase over the Castle class, necessary to accommodate the new WA designated boiler. The driving and bogie wheels were reduced in size from that of the Castles and the boiler working pressure increased to 250lb, with a larger diameter cylinder and a longer stroke. The Great Western promoted the new Kings as “the most powerful passenger locomotives in the country” and at 29ft 5in and weighing 89 tons, the class probably represented the limit of the 4-6-0 design.