The locomotives were designed by Nigel Gresley to haul express trains over the difficult Edinburgh to Aberdeen section of the London and North Eastern Railway.

In the design Gresley was influenced by recent French practice, in particular passenger locomotives of the Paris à Orléans railway.

It was built at Doncaster Works,[4] with Lentz-type[citation needed] rotary-cam actuated poppet valve-gear supplied by the Associated Locomotive Equipment Company,[5] and a double-chimney Kylchap exhaust, each chimney using four nozzle blastpipes.

10000, derived from Dr. Dalby's wind tunnel research, and the attached tender was of the standard design used on Gresley Pacifics.

2002 Earl Marischal was completed by 1935, also at Doncaster, and was fitted with Walschaerts valve gear, as used on Gresley Pacifics, and had a greater superheater heating area of 776.5 sq ft (72.14 m2), obtained by using larger diameter fire tubes.

2002 proved to be more efficient than 2001, due to a lower cylinder clearance volume and because the stepped-cam cutoff modifications made to No.

2004 was fitted with an experimental butterfly valve blastpipe bypass, manually activated to prevent fire-lifting at high cutoffs.

This was later replaced with a plug valve and higher bypass pipe diameter, but both designs had problems with sticking due to carbonised deposits.

2004 Mons Meg on the early morning 'Aberdonian' non-stop from Edinburgh to Dundee – a train with gross weight of 320 tons.

Despite the vehicle's long wheelbase (19 ft 6 in (5.94 m)), and the frequent curves on the route, Livesay did not report any serious problems with ride quality.

[24] Further Edinburgh to Dundee and return runs were made in the afternoon with 355 and 450 ton gross loads, recording average speeds of 40.25 and 33 mph respectively.

The locomotive passed Finsbury Park without slipping and reached Peterborough, Grantham, Doncaster, York, Darlington, Newcastle, Edinburgh, Dundee and Aberdeen all ahead of schedule.

[28] According to Bert Spencer, a senior LNER engineer, the class was rebuilt due to reliability problems during the difficult conditions of the World War II period, and additionally to take the opportunity to try out a different valve gear arrangement.

[30] Cecil J. Allen supported such a view, believing the engines would have been welcomed on services between King's Cross and Newcastle.

[31] Oliver Bulleid was quoted that the class had been placed into services in which they were under-utilised or misused, leading to poor fuel economy.

[32][33] Railway author O. S. Nock suggested that Gresley's successor, Edward Thompson, may have made largely unsubstantiated criticisms of the class in order to justify the rebuilding.

[38] Modern modifications to the original design include roller bearings (also featured on Tornado) and an all-welded, all-steel boiler; the final build will utilise Lentz poppet valve gear.

2001 Cock O' The North[41] as modified in 1938, using Gresley motion, Walschaerts valve gear, and an A4 style 'Bugatti' streamlined front.

[44] In 2013, Hornby Railways collaborated with the A1 Locomotive Trust to produce models of the class, starting with the original Cock O' The North, for release in early 2014.