Since at least as early as World War I, the U.S. Army had been looking for a fast, lightweight all-terrain reconnaissance vehicle. In early 1940, however, things became urgent as the Axis powers began to score victories in Europe and Northern Africa and there became the need to rapidly develop a new type of vehicle. The Army put out a call to automobile manufacturers asking for a running prototype for such a vehicle in just 49 days.

The original government specifications were as follows: Vehicle weight: Approximately 1,300 pounds (This proved to be totally unrealistic and later was raised to 2,160 pounds.)
Four-wheel drive
Engine (power): 85 pound-feet of torque
Wheelbase: Not more than 80 inches
Tread: Not more than 47 inches
Ground Clearance: Minimum ground clearance of 6.25 inches
Payload: 600 pounds
Cooling System: Good enough to allow a sustained low speed without overheating the engine.

The Bantam Car Company, which had supplied some earlier reconnaissance vehicles to the Army, and Willys-Overland, were the only two companies that responded to the Army's call, although over 130 companies had been invited to respond. The 49-day deadline was problematic, however, and Willys-Overland asked for more time to finish their vehicle. Bantam's only hope to meet this deadline was to bring in outside help.

The prototype of the vehicle that would become the Willys MB was designed by Karl Probst, a Detroit engineer who had worked for several automotive firms. Enlisted by National Defense Advisory Committee Head William S. Knudsen (former president of General Motors), Probst accepted the patriotic challenge without salary and went to work July 17, 1940. In just two days, he had completely laid out plans for the Bantam prototype, the precursor of the Jeep® vehicle. On July 22, Bantam's bid was submitted, complete with layouts of this new vehicle. The bid claimed that the vehicle met the weight limit of 1,300 pounds, although it was actually much heavier.

Bantam's first hand-built prototype was complete and running by September 21, 1940, meeting the 49-day deadline. The Army put this prototype through torturous testing, taking the Bantam Jeep vehicle over 3,400 miles, all except about 250 were unpaved. The testers eventually concluded "this vehicle demonstrated ample power and all requirements of the service."

Ultimately, both Willys and Ford submitted prototypes based on the Bantam plans supplied to them by the Army. The Willys "Quad" and the Ford "Pygmy" prototypes added their own changes and modifications to the basic Bantam design.

For example, the Willys Quad prototype exceeded the specified weight limit, due in large part to its superior engine. This ultimately worked to Willys' advantage when the weight limit was increased: the strength in the Willys vehicle - powered by its "Go Devil" - was the only one that met the Army's power specifications. In fact, the Willys' 105 pound-feet of torque not only exceeded the required power, but dwarfed Bantam's 83 and Ford's 85 pound-feet of torque.

In light of Bantam's shaky manufacturing and financial position, and the advantages of the Willys vehicle, the Army contract was awarded to Willys. Since the War Department required a large number of vehicles to be manufactured in a relatively short time, Willys-Overland granted the United States government a non-exclusive license to allow another company to manufacture vehicles using Willys' specifications. Pursuant to this agreement, Willys supplied Ford Motor Co. with a complete set of specifications.

During World War II, Willys and Ford filled more than 700,000 orders, with Willys-Overland supplying more than 330,000 units of the original Willys MB.