The class was introduced by Nigel Gresley in July 1926, based on his previous Class J38 (introduced in January 1926) but with larger driving wheels. The larger wheels enabled them to be used on both passenger and freight trains, although at the expense of a lower tractive effort. As a result they were given the BR power classification 4P/5F, rather than the 6F of the earlier class. A total of 289 examples were built over next fifteen years.
The larger wheels necessitated the provision of low splashers over the front two wheels, which is the main means of differentiating between the two classes. The locomotives were all fitted with superheaters and Ross 'Pop' safety valves.
J39/2 No. 64898 at Lincoln, Pelham St. Level-Crossing 21 May 1956
All passed into British Railways ownership in 1948 and they were numbered 64700-64988. They began to be withdrawn from service in 1959 and all examples had been scrapped by the end of 1962.