The Big Boy class was developed by Union Pacific (UP) and the American Locomotive Company (ALCO) to handle the 1.14% eastbound ruling grade of the Wasatch Range. UP determined that its goals for the new class could be achieved by making several changes to the existing 4-6-6-4Challenger design by enlarging the firebox to about 235 by 96 inches (5.97 m × 2.44 m) (about 155 sq ft or 14.4 m²), lengthening the boiler, adding four driving wheels and reducing the diameter of the driving wheels from 69 to 68 in (1,753 to 1,727 mm).

The Big Boy was articulated like the Mallet locomotive design, although without compounding. It was designed for stability at 80 miles per hour (130 km/h), allowing for a wide margin of reliability and safety, as steam locomotives normally operated well below that speed in freight service. Peak horsepower was reached around 35 mph (56 km/h); optimal tractive effort was reached around 10 mph (16 km/h). It is longer than two city buses and weighs more than a Boeing 747.