When the North British Railway needed new express locomotives which were much more powerful than the existing 4-4-0, William Reid developed the Atlantic that was later called class H. It was the only five-axle tender locomotive of this railway and also the most powerful one. The decision not to design a ten-wheeler and to use only two driving axles may have been influenced by the tight curves in the NBR network.

The first batch of 14 was built by the North British Locomotive Co. in 1906. Although they initially had problems with mass balancing and were too long for the existing turntables, they quickly became the flagship of the NBR. When it was realized that there was the need for more of these powerful locomotives, six more were built by Robert Stephenson & Co. in 1911. Two additional locomotives were built in 1925 by North British, which were equipped with superheaters. 

All remaining locomotives were superheated starting in 1915. To distinguish them, the non-superheated locomotives were renamed to class I and renamed back to H when the superheater was installed. This process took until 1926, so even the LNER used a similar approach by calling the last non-superheated ones class C10 while the others were class C11.

After the introduction of the LNER Pacifics, the Atlantics were used for less important trains. They were finally withdrawn between 1933 and 1937. The “Midlothian” was selected for preservation, but since the order to preserve it came too late, it was already partially scrapped. Although it was rebuilt, the steel shortage in World War II sealed its fate. So it was cut up and the material was used for the production of aircraft.