Incorporating a number of new developments in British steam locomotive technology, the design of the Merchant Navy class was among the first to use welding in the construction process; this enabled easier fabrication of components during the austerity of the war and post-war economies.

[4] Due to problems with some of the more novel features of Bulleid's design, all members of the class were modified by British Railways during the late 1950s, losing their air-smoothed casings in the process.

Following the retirement of the general manager of the Southern Railway Sir Herbert Walker and Richard Maunsell the Chief Mechanical Engineer (CME) in 1937, their successors considered that the time had come to change this situation.

[10] In March 1938 the new general manager Gilbert Szlumper authorised Oliver Bulleid, Maunsell's replacement, to prepare designs for twenty express passenger locomotives.

[19] It later gained a bad reputation, because it could cause highly irregular valve events, a problem compounded by the fast-moving Bulleid steam reverser.

[20] The locomotives were equipped with the unusual 6 ft 2 in (1.88 m) Bulleid Firth Brown (BFB) driving wheels which both lighter and stronger than the spoked equivalent.

[22] A long coupled driving wheelbase was incorporated into the design, to keep the locomotives within the lineal loading of the Southern Railway's narrower bridges.

[22] The supporting rear trailing truck was a one-piece steel casting that gave the smoothest of rides; the design was utilised in the future BR Standard Class 7.

[21] The boiler was enveloped by Bulleid's air-smoothed sheet-steel casing, which was not for the purposes of streamlining, as demonstrated by the extremely flat front end, but as a way of lifting exhaust gases.

[23] The flat sides were also an aid to cleaning the locomotive with mechanical carriage washers, representing an attempt to reduce labour costs.

[22] It featured BFB wheels and streamlined panels, or "raves", that gave the top of the tender a similar cross-sectional outline to the carriages hauled by the locomotive.

[20] The water tank was of welded sheet construction to save weight, and the tender was fitted with vacuum braking equipment of a clasp-type similar to that used on the locomotive.

The original tender design proved to be inadequately braced and subject to serious leakage if even slightly damaged, or when water surges caused the welded joints to split.

[26] As an aid to the fireman, a steam-operated treadle was provided that used steam pressure to open the firehole doors (where the coal is shovelled into the firebox).

Both prototypes were found to be seven tons over the specified weight, and, at the insistence of the Southern Railway Civil Engineer, production of the remainder was halted until steps were taken to remedy this.

[29] This precipitated scale-forming constituents in the "hard-water" of southern England into a non-adhesive mud that could be cleared from the locomotive using a manual "blow-down" valve.

Bulleid adopted a new numbering scheme for all his locomotives based on Continental practice, following his experiences at the French branch of Westinghouse Electric before the First World War, and those of his tenure in the rail operating department during that conflict.

[30] The Southern Railway considered naming the locomotives after victories of the Second World War, to the extent that a mocked-up nameplate River Plate was produced.

[31] In the event, when early successes for the British proved few and far between, the chairman of the Union-Castle Line suggested naming them after shipping companies which had called at Southampton Docks in peacetime.

During their operational career, the class gained several nicknames; the most obvious, Bulleid Pacific, simply denoted the designer and wheel arrangement.

[10] In August 1945, a series of test runs were made between London Victoria and Dover and from October the class were used on the resumed Continental expresses.

The steam-operated firehole door treadle was removed, and a new type of boiler cladding was utilised in response to the worsening supply situation during the Second World War.

[36] During the time they operated under the Southern Railway, further modifications were applied to the class, such as the reduction in boiler pressure to 250 psi (1.72 MPa) and the redesign of the footplate spectacle plates.

[3] Originally, the spectacle plates of the Bulleid Pacifics were at the conventional right-angle to the direction of the locomotive, and offered limited vision ahead along the air-smoothed casing.

[38] The Southern-built batches also had variations in the material used for the air-smoothed casing with a change from sheet steel to an asbestos compound, forced upon the manufacturer by wartime expediency.

[41] The resultant 'cut-down' tender included new, enclosed storage for fire-irons, revised step ladders and glass spectacle plates to protect the crew from flying coal dust when running tender-first.

[51] The fast-moving and unpredictable Bulleid steam reverser was replaced with a screw-link version, whilst the mechanical lubricators were moved to the footplates along the boiler sides.

[54] There is no doubt that rebuilding the class solved most of the maintenance problems, whilst retaining the good features, thereby creating excellent locomotives.

[69] All class members that operated during the Second World War were eventually repainted in Southern Railway wartime black livery, with green-shaded "Sunshine" lettering.

The utilisation of welded steel construction and the various innovations that had not previously been seen in British locomotive design meant that the class earned Bulleid the title "Last Giant of Steam".