Willys-Overland was one of several bidders when the War Department sought an auto maker that could begin rapid production of a lightweight truck based on a design by American Bantam.

Production of the Willys MB, better known as Jeep, began in 1941, shared between Willys, Ford and American Bantam. 8,598 units were produced that year, and 359,851 units were produced before the end of World War II. Willys-Overland ranked 48th among United States corporations in the value of World War II military production contracts. In total, 653,568 military Jeeps were manufactured.

By July 1941, the War Department desired to standardize and decided to select a single manufacturer to supply them with the next order for another 16,000 vehicles. Willys won the contract mostly due to its more powerful engine (the "Go Devil") which soldiers raved about, and its lower cost and silhouette. The design features the Bantam and Ford entries had which were an improvement over Willys' were then incorporated into the Willys car, moving it from an "A" designation to "B", thus the "MB" nomenclature. Most notable was a flat wide hood, adapted from Ford GP.

By October 1941, it became apparent Willys-Overland could not keep up with production demand and Ford was contracted to produce them as well. The Ford car was then designated GPW, with the "W" referring to the "Willys" licensed design. During World War II, Willys produced 363,000 Jeeps and Ford some 280,000. Approximately 51,000 were exported to the U.S.S.R. under the Lend-Lease program.