One of George Jackson Churchward's first designs for the GWR was a 2-6-2T passenger tank locomotive intended for suburban service. The Prototype number 99 was completed in 1903 and formed the basis of a family of similar locomotives built through 1949, nicknamed “Large Prairies” to distinguish them from the GWR's smaller 2-6-2Ts. As with most of the locomotives in this family, the No. 99's driving wheel diameter was five feet and eight inches. This allowed sufficiently high speeds in suburban service, but at the same time adequate pulling power was still available. With outside cylinders, it was modern for a British locomotive of the time, which made a larger cylinder diameter possible using the GWR's loading gauge. 

The first batch consisted of 39 examples, all of which were delivered in 1903. These differed from the prototype in that the tops of the water tanks were sloped slightly at the front to improve visibility. After getting the numbers 3111 to 3149, also 99 was renumbered to 3100 in 1912. As a result, the locomotives were now known as class 3100, but this changed again in 1927 with the renumbering to 5100 and 5111 to 5149.

In 1938 and 1939 the prototype and nine other production locomotives were rebuilt with two inch smaller drivers and a new boiler with a pressure of 225 instead of 200 psi. These achieved a slightly higher tractive effort and were listed as class 8100. However, since the increase in tractive power was negligible during operation, no further locomotives were rebuilt and the class 8100 was used together with the class 5100.