In the twenties, the Great Northern needed powerful freight locomotives for the more than 900 mile-long stretch from Minot, North Dakota through Montana to Spokane, Washington. They needed lots of pulling power for the grades over Marias Pass, but enough speed for the long distance. The first four locomotives of class R-1 were delivered by Baldwin in 1925. They had the 2-8-8-2 wheel arrangement, four equal cylinders of 28 by 32 inches (711 by 813 mm) and drivers of 63 inches (1,600 mm).

At their boiler pressure of 210 psi, this resulted in a starting tractive effort of 142,165 pounds when using the usual formula. But since the Great Northern used a coefficient of .765 instead of .85, they only quoted a value of 127,950 pounds. The six-axle Vanderbuilt tender carried 16,800 gallons of water and 5,800 gallons of oil. Together, the locomotive and tender reached a total weight of nearly 450 tons.

Based on the first four R-1, 10 more were built in the GNR'S own Hillyard shops in 1927 and 1928. But these were not only heavier, but had their pressure raised by 15 psi and got a bigger firebox heating surface. While the Baldwin locomotives only had a combustion chamber, the new ones also got arch tubes which added 64 square feet. Also the older ones later got the arch tubes.

Since their operator still needed more power, they built 16 of the class R-2 in 1929 and 1930. The water capacity of the tender was increased to 22,000 gallons. At a total weight with tender of more than 500 tons, they were the biggest US locomotives built west of the Mississippi. The boiler pressure was again increased by 15 psi, the grate area was increased and the superheater was made much larger.

By 1946 they had been upgraded by increasing the firebox heating surface by 142 square feet through the addition of circulators and by an increased weight that led to a 24 tons higher adhesive weight. The combined surface of firebox, tubes, flues and superheater now reached a value of 11,349 square feet (1,054 m²). A special measure was the reduction of the diameter of only the front cylinders by two inches to match the steam consumption with the boiler.