The Pennsylvania Railroad's Juniata Shops in Altoona, PA, outshopped a single "Mountain" 4-8-2 locomotive in 1923. It was assigned road number 4700 (later is was renumbered 6699) and was designated Class M1. It was a common practice for the PRR to build a prototype that could be tested thoroughly before ordering more of a locomotive design. It tested the Class M1 prototype for two years before approving it for fleet construction.

The design of the PRR's "Mountain" locomotive was not an extension of its Class K4s "Pacific", but rather a descendant of its I1s "Decapod". It was the work of J. T. Wallis, Chief of Motive Power and W. F. Kiesel, Jr, Mechanical Engineer, who started with an I1's boiler, enlarged the combustion chamber, then used the "Decapod's" guides, crossheads and many other parts.

In 1925, the PRR ordered 200 Class M1 "Mountains", 175 (road numbers 6800 through 6974) from the Baldwin Locomotive Works and 25 (road numbers 6975 through 6999) from the Lima Locomotive Company. These locomotives had 72" drivers, 27 x 30 cylinders, a 250 psi boiler pressure, a tractive effort of 64,550 lbs and weighed 385,000 pounds.

One hundred, Class M1a "Mountains" were added to the roster in 1930, fifty (road numbers 6700 through 6749) from Baldwin, twenty (road numbers 6777 through 6799) from Lima and twenty (road numbers 6750 through 6774) built in the Juniata Shops. Class M1a locomotives were very similar to the Class M1s but, included two air compressors instead of one and had Worthington feedwater heaters.

There is one PRR Mountain survivor, number 6755, at the Railroad Museum of Pennsylvania in Strasburg, PA.