In 1935, William Stanier, Chief Mechanical Engineer of the LMS, ordered the rebuilding of the unique experimental high pressure compound locomotive 6399 Fury.
[2] On testing the boiler performed poorly, with heat transfer to the water being inadequate because the hot gases from the firebox passed too rapidly through the tubes to the smokebox.
[2] In 1935, a design for an improved taper boiler was schemed out; this was 13 ft (4.0 m) long, with 198 tubes 1+3⁄4 inches (44 mm) in diameter.
[3] Combined with modifications to the steam ports and valve gear, the rebuilding produced locomotives that were more powerful and economical than either the original Jubilees or the Royal Scots.
[4] As the boilers of the Royal Scots were due for replacement, and their built-up smokeboxes were proving difficult to keep airtight, the decision was taken to rebuild the class in batches.