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.