In 1926, with increased loading on the East Anglia passenger services and the introduction of modern, vacuum-braked coaching stock, there was a desperate need for a locomotive that could supplement the Holden B12 Class on the former Great Eastern routes of the London North Eastern Railway. Track limitations prevented the transfer of locomotives from other regions and so Nigel Gresley was tasked by the LNER to produce a three cylinder 4-6-0 design, using the cylinder and motion arrangement of the D49, but with a tractive effort of about 25,000lb and a relatively light axle loading of 17 tons.
The resulting Class B17 design arose from Doncaster Works’ inability to completely satisfy Gresley's specification and the resulting contract for the detailed design and building of the class was, in 1927, given to the North British Locomotive Company in Glasgow. Using some of the features from a batch of A1 Pacific locomotives they had built in 1924, along with some features from the Royal Scot Class produced for the LMS, the cab, cylinders, and motion were all copied directly or modified slightly, whilst most of the boiler design was taken from the Class K3 and Class O2 designs. Darlington Works assisted by providing drawings for the bogies, with Stratford Works providing the design for the Great Eastern type 3,700 gallon tender.