For these respective pre-production runs, each vehicle received revisions and a new name. Bantam's became the BRC 40, and the company ceased motor vehicle production after the last one was built in December 1941. After losing 240 pounds of weight, Willys' changed the designation to "MA" for "Military" model "A". The Fords went into production as "GP", with "G" for a "Government" type contract and "P" commonly used by Ford to designate any passenger car with a wheelbase of 80 inches.

The Army designated 1/4-ton 4x4 Truck built by Willys-Overland were Model MB, while vehicles built by Ford were Model GPW (G = government vehicle, P designated the 80" wheelbase, and W = the Willys engine design). There were subtle differences between the two.[4] The versions produced by Ford had every component (including bolt heads) marked with Ford logos. Willys also followed the Ford pattern by stamping its name into body parts, but stopped this in 1942.[5] The cost per vehicle trended upwards as the war continued from the price under the first contract from Willys at US$648.74 (Ford's was $782.59 per unit).

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. Whatever better design features the Bantam and Ford entries had were then incorporated into the Willys car, moving it from an "A" designation to "B", thus the "MB" nomenclature.

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 USSR under the Lend-Lease program.

A further 13,000 (roughly) amphibian jeeps were built by Ford under the name GPA (nicknamed 'Seep' for Sea Jeep). Inspired by the larger DUKW, the vehicle was produced too quickly and proved to be too heavy, too unwieldy, and of insufficient freeboard. In spite of participating successfully in the Sicily landings (July 1943) most GPAs were routed to the USSR under the Lend-Lease program. The Soviets were sufficiently pleased with its ability to cross rivers to develop their own version of it after the war.