 Trials undertaken in 1914 with the H15 class prototype had demonstrated to Urie that the basic design showed considerable speed potential on the Western section of the LSWR from Basingstoke westwards, and could form the basis of a powerful new class of 4-6-0 express passenger locomotive with larger 6 ft 7 in (2.01 m) driving wheels. The LSWR required such a locomotive, which would need to cope with increasing train loads on this long and arduous route to the West Country. The result was the N15 class design, completed by Urie in 1917. It incorporated features from the H15 class, including eight-wheel double bogie tenders with outside plate frames over the wheels and exposed Walschaerts valve gear. High running plates along the boiler were retained for ease of oiling and maintenance.
