This was the second class to be built at the new Kilmarnock Locomotive Works, in two batches. The first six locomotives numbered 40, 39, 27, 10, 43 and 42 (Works Nos. 14–19) were built between 1860 and January 1862. A further four locomotives, numbered 28–9, 31 and 44 were built in 1864 (Works Nos. 26–29).
The members of the class were fitted with domelessboilers. They were originally built without cabs, but several later had them fitted. Two members of the class were fitted with Giffardinjectors.