Charles Benjamin Collett, GWR's chief engineer since 1922, soon had the task of developing a more powerful express locomotive than the Castle class of its predecessor Churchward. The goal was to achieve a starting tractive effort of 40,000 pounds, despite the high speeds. First, the question of the ideal wheel arrangement had to be examined.

While the Pacific had long established itself as the ultimate express locomotive on the European mainland, the ten-wheeler remained largely the choice in Great Britain. Although the GWR had already built a single Pacific in 1908 with “The Great Bear”, this design was considered prone to slipping. After all, only half of the axles on the Pacific were powered, compared to 60 percent on the Ten-wheeler. So the choice fell again on a ten-wheeler, but with an axle load of 22 and a half tons.

Much of the power increase over the Castle class was achieved by increasing the boiler pressure to 250 psi. The boiler was introduced as GWR standard design no. 12, but only used on the King class. With a tube length of only 16 feet, it no longer reached the excessive length of “The Great Bear” and was able to achieve greater power using high quality coal from South Wales.

In addition, the stroke of the four cylinders has been increased by two inches and the diameter of the coupled wheels has been reduced by 2.5 inches to a still adequate 6 feet 6 inches. Since the required 40,000 pounds of starting tractive effort could not quite be achieved, the cylinder diameter was increased by a quarter of an inch.

No. 6001 “King George V”

Railway and Locomotive Engineering, July 1927

Initially, the new class was to be named after cathedrals. However, when number 6000 was brought to the United States for the Baltimore & Ohio Jubilee celebrations in 1927, it was christened “King George V” after the current regent. As a result, all other engines were named after British kings. After a total of 20 production engines that had been manufactured in 1927 and 1928, another ten followed in 1930. After No. 6007 “King William III” was destroyed in an accident in 1936, a new loco of the same number and name was built using some of the assemblies that were still intact.

The King class was used in front of the main express trains of the GWR, which ran west towards Cornwall and sometimes consisted of more than 13 cars. A second locomotive only had to help on the steep inclines in Devon. From Plymouth at the latest, however, another locomotive had to be used for the rest of the route through Cornwall, since the axle load of the King class was too high for the Royal Albert Bridge. It also turned out that the outer cylinders with a diameter of 16.25 inches were too wide for the loading gauge in some places, so that they later had to be converted to 16 inches.

The later modifications to individual pieces were limited. For example, in 1935, road number 6014 “King Henry VII” was partially streamlined. Only the superheaters were replaced by a different design from 1947 onwards. When the locomotives were distributed to other parts of the country when British Railways was founded in 1948, it turned out that they showed a noticeable loss of power elsewhere with less energetic coal. At the end of the 1950s they were still equipped with a double induced draft system, but they were all phased out by 1962.