During World War II the Pennsylvania Railroad needed heavier locomotives to pull freight and military equipment, but wartime restrictions prohibited the development of a new locomotive design. In response to this the Pennsylvania Railroad borrowed a 2-6-6-4 Class A of the Norfolk & Western Railwayand a 2-10-4 from the Chesapeake and Ohio Railroad. Both locomotives underwent extensive testing, with the C&O 2-10-4 chosen to be produced. A total of 125 were built at PRR's shops in Juniata, Pennsylvania. They came to be known as the PRR's "War Babies," but the J1's remained in service into the 1950s. When the Pennsylvania Railroad converted from steam power to diesel, the PRR scrapped most of them in 1958 with the exception of 25. The remaining 25 were scrapped in 1959.

The PRR J1 was a class of 2-10-4 "Texas" type steam locomotives with 69 in (180 cm) driving wheels built between 1942 and 1944. The J1 had over 95,000 pounds-force (422.6 kN) of tractive effort, plus an additional 15,000 lbf (66.7 kN) if the booster engine was used.