The class R-1 Mountains were the last steam locomotives ordered by the Boston & Maine. They were among the heaviest Mountains and were built in four batches between 1935 and 1941. Initially five R-1a were built, in 1937 another five R-1b, in 1939 three R-1c and finally five R-1d. All locomotives were given names that were determined in a competition.

From the R-1b onwards, the spoke wheels on the driven axle were replaced with disc wheels. The R-1c was almost identical to the R-1b, only the R-1d underwent major changes. It now had disc wheels on all drivers and the Coffin feed water heater was replaced with a Worthington one. In addition, there was now a seven-axle tender instead of the six-axle one. Thirteen R-1s were sold to the B&O in 1947, the remaining five were scrapped in 1955 and 1956.