The Manor class was designed in the late thirties when the GWR found out that the axle load of the mixed-traffic Grange class was too high for many lines. Therefore Charles Collett had the lighter Swindon No. 14 boiler designed, combined with a cylinder diameter that had been decreased by half an inch compared to the Grange. As with the Grange, the driving wheels, many motion components and the tenders were taken from old class 4300 Moguls. In the result the axle load had been reduced by more than a ton. After the first batch from 1938 and 1939, British Railways built another batch of ten in 1950. They were numbered 7800 to 7829 and named after Manors in the GWR area.