They were not a new design, being a straightforward development of the 120-strong 1854 Class dating back to 1890. This lineage had begun with George Armstrong's 645 Class in 1872 and continued via the 1813 Class (in 1882), the 1854 Class, and finally ended in 1948 with the GWR 9400 Class. The differences over 76 years were undeniably a gradual evolution, with increases in boiler pressure and heating surface the more important, enclosed cabs and larger bunkers cosmetic but functional. The biggest change was the fitting of a Belpaire firebox necessitating a pair of pannier tanks, as the square-topped firebox is not compatible with a curved saddle tank. Highlighting the gradual nature of the changes are the subtle differences between the 1854 and 2721 classes, confined to a small increase in wheel size by 1½ inches, fluted rods and coil springs all round, at least when new.
When later rebuilt with pannier tanks they were the direct predecessors of the GWR 5700 Class of 1929. 43 of them survived into British Railways ownership, by which time all had been rebuilt with pannier tanks. Their numbers were 2721-2800 (the last being renumbered 2700 in 1912 to make room for the pioneer 2-8-0GWR 2800 Class). Withdrawals started in 1945 and were complete by 1950 by which time the last of the replacement 57xx's (in the 96xx series) were in service.