The Castle Class was designed by Charles Collett, the successor to legendary GWR engineer George Jackson Churchward. In truth, Castle Class is nothing more than a nickname owing to most examples of the class being named after western castles with the official GWR designation being the 4073 class.
This name comes from the first example of the Class 4073 Caerphilly Castle, as was common with GWR classes. Charles Collet improved on the work of Churchward with the Castle Class, with the class proving to be among the most powerful on the GWR network. In a tragic twist of fate, Churchward would go on to be killed when struck by one of the succeeding engineers locomotives 4085 Berkeley Castle.
The first of the class would feature at the British Empire Exhibition in 1924 alongside an equivalent engine from the LNER, one 4472 Flying Scotsman. Over the course of the exhibition the GWR claimed their Castle to be more powerful than the LNER contingent, leading to exchange trials between the GWR and image conscious LNER. In April 1925 4079 Pendennis Castle and 4474 Victor Wild would be exchanged between the two companies.
The Castle Class was lighter than the LNER pacific which proved to be an unexpected positive as the LNER found it still capable of hauling their express services doing so faster and more efficiently than their locomotives of the time. Conversely, the GWR found the LNER locomotive to have too long a wheelbase for their rails, hampering is performance on the twisting and winding curves of the western region.