The design and construction took place at the ex-Great Western Railway Swindon Works, along with the 2-6-2 tank engine version of the class, though some details were designed at Brighton, Derby and Doncaster. Although the boiler shared flanged plates with the GWR No.2 boiler the barrel was shortened by  inches and a dome added. Strangely the class did not share the same design of wheels as the Doncaster-designed BR Standard Class 4 2-6-0, however both shared the driving wheels and the same piston stroke, and hence crank-pin throw.