Novel features included an all-time high boiler pressure of 280lb p.s.i., clasp brakes, welded steel firebox with thermic syphons, steam operated fire doors (novel in Britain), a steam driven electrical generator providing comprehensive headcode, inspection and cab lighting, a cab layout permitting the driver and fireman to work without getting in each other's way, but chief of all a totally enclosed motion encased in an oil bath situated between the frames. This motion itself included the new feature of a chain driven three throw crank shaft operating valve gears for each cylinder. In his motion gear Bulleid was strongly influenced by automobile design with its potential increased reliability and reduced maintenance compared to conventional locomotive motion design.

The "Southern" plate on the smokebox door was unpopular with loco crews as it resembled an upside-down horseshoe so was soon replaced with a "Southern Roundel" with the added section carrying the date of the manufacture. Despite a wooden mock-up being made immediately after nationalization, the roundel was not replaced with a similar one bearing the legend "British Railways". In the early days of BR the numbers, by now simply painted, carried the prefix 'S' in front of the Southern number, but when the locos were renumbered in the 350xx series, this was replaced by a cast LMS-style plate on the smokebox door, hidden in the photograph below by the train headboard.

It was originally intended to name this class after Allied victories. Nº21C1 had been lined up for the name The Plate but in 1941 there had been few victories to commemorate. Consideration was then given to naming them after Commonwealth capitals but the Chairman of the Union Castle Line then suggested naming them after shipping companies which had called at Southampton Docks in peacetime.

The first member of the class - Nº21C1 Channel Packet - was completed at Eastleigh Works in February 1941 - design and construction having been overtaken by the outbreak of World War Two. Two further batches of ten were ordered, the final batch in the early days of nationalisation which never carried Bulleid notation numbers. With a tractive effort of 37500lb at 85% boiler pressure the Merchant Navy class provided the Southern with a modern powerful express passenger locomotive. Three members of the class, Nº35017 Belgian Marine, Nº35019 French Line C. G. T. and Nº35020 Bibby Line (the reserve engine), all temporarily matched with higher capacity LMS water scoop tenders, took part in the British Railways locomotive exchange trials of 1948 in the express passenger class trials where they more than held their own.

However in the early days the class experienced persistent teething troubles whilst in their original condition some of the novel features failed to deliver their original labour and cost saving intentions. The steam reverser and the oil bath enclosed motion gave particular problems. It proved impossible to keep the oil bath sealed with the result that the boiler lagging became oil soaked and prone to catching fire, and the class was also known for its slippery starts. The air-smoothed casing also caused a maintenance headache, and they were inefficient users of coal and oil.