Without any extensive prototype trials, one hundred and thirteen 3-cylinder 4-6-0 express engines designed by W.A. Stanier were ordered in 1933, and ultimately one hundred and ninety were built.
The engines were intended to be an improved taper boiled version of the 'Patriot' Class, having small superheaters and domeless boilers like the original Class 5's and the first of the Class No.5552, when new in 1934. Unlike the Class 5, they were an immediate disappointment. they were usually short of steam and the low temperature superheater robbed them of much of the 'vivacity' of the Patriots.
In 1935, 5642 exchanged numbers with 5552, was finished in black enamel, with cast, chromium-plated letters and numerals, complemented by other chromium-platted embellishments. She was named 'Silver Jubilee', which gave the name to the Class.
Extensive trials showed that suitable alterations to blast pipes and chimney dimensions greatly improved the steaming capacity of the boiler while later engines were built with regulaters in the dome, larger super heaters and bigger grates. All the earlier engines later received these improved boilers.
As well as the alterations made to the single blast pipe arrangements of the Class, double blast pipes and chimneys were tried on two of the engines. As these were designed and fitted before the work on draughting had been carried out, the results showed showed little improvement on the modified single blast pipes.
Engines originally received Fowler 3,500 gallon tendres, later to be replaced with the 4,000 galon Stanier variety.