Indeed he was set a tough challenge for this specification by the Chief Operating Manager because the future standard for main line express trains was to haul a load of 500 tons tare at a start-to-stop average speed of 55 m.p.h. on not only the South West section but also over the demanding to operate Eastern section. Maunsell set about this in a systematic fashion with development work and trials using modified locomotives, and evaluating and comparing the designs of other companies with the aim of producing what would be called today a "state of the art" locomotive. However first there was a pressing need for more express passenger locomotives for the expanded summer timetable of 1925 and this was met by construction of additional class N15 engines with an improved front end.

After considering various options the eventual specification called for a four cylinder 4-6-0 locomotive with cranks set at 135 degrees, the drive divided between the first two coupled axles, 6 ft 7 in diameter driving wheels, a boiler pressure of 220 lb psi and a tractive effort of 33500 lb - the highest of any British express passenger locomotive of the time. One original feature was the settings of the cranks led to eight power impulses per revolution in place of the normal four, leading to eight puffs to be heard for every turn of the wheels! This applied to all except the last of the class, 30865, which has the more normal four exhaust per revolution. A Belpaire firebox was employed and although a longer boiler had been considered earlier a boiler with tubes the same length as those employed on the N15 class was used. The fire grate was to be unique to the Southern (but not to another railway associated with Swindon) with the rear portion being horizontal and the front steeply inclined providing an area of 33 sq ft, the largest of any British locomotive of the time (although later exceeded by the GWR King class). Because the four cylinder design threatened to result in a locomotive heavier than the Civil Engineer's axle loading limit special care was taken in design and construction to keep the locomotive's weight to a minimum. High tensile steel was employed for the motion. Parts which would normally have been left as cast or forged were machined to remove excess metal and the frames were made as thin as practical with additional lightening holes. When constructed the prototype locomotive came out only 1 ton 1 cwt heavier than the N15 class so production models did not employ the latter two weight reducing factors. An increase of almost 33 percent tractive effort for only just over a 1 ton weight gain between the two classes is a tribute to the skilful design.