The Willys MB (/ˈwɪlɪs/ Willis) and the Ford GPW, both formally called the U.S. Army truck, 1⁄4‑ton, 4×4, command reconnaissance, commonly known as the Willys Jeep, Jeep, or jeep, and sometimes referred to by its Standard Army vehicle supply number G-503, were highly successful American off-road capable, light military utility vehicles. Well over 600,000 were built to a single standardized design, for the United States and the Allied forces in World War II, from 1941 until 1945. This also made it the world's first mass-produced four-wheel-drive car, built in six-figure numbers.

The 1⁄4-ton jeep became the primary light, wheeled, multi-role vehicle of the United States military and its allies. With some 640,000 units built, the 1⁄4‑ton jeeps constituted a quarter of the total military support motor vehicles that the U.S. produced during the war, and almost two-thirds of the 988,000 light 4WD vehicles produced, when counted together with the Dodge WC series. Large numbers of jeeps were provided to U.S. allies, including the Soviet Union at the time. Aside from large amounts of 11⁄2- and 21⁄2‑ton trucks, and 25,000 3⁄4‑ton Dodges, some 50,000 1⁄4‑ton jeeps were shipped to help Russia during WWII, against Nazi Germany's total production of just over 50,000 Kübelwagens, the jeep's primary counterpart.

Historian Charles K. Hyde wrote: "In many respects, the jeep became the iconic vehicle of World War II, with an almost mythological reputation of toughness, durability, and versatility." It became the workhorse of the American military, replacing horses, other draft animals, and motorcycles in every role, from messaging and cavalry units to supply trains. In addition, improvised field modifications made the jeep capable of just about any other function soldiers could think of. Military jeeps were adopted by countries all over the world, so much so that they became the most widely used and recognizable military vehicle in history.

Dwight D. Eisenhower, the Supreme Commander of the Allied Expeditionary Force in Europe in World War II, wrote in his memoirs that most senior officers regarded it as one of the five pieces of equipment most vital to success in Africa and Europe. General George Marshall, Chief of Staff of the US Army during the war, called the vehicle "America's greatest contribution to modern warfare." In 1991, the MB Jeep was designated an "International Historic Mechanical Engineering Landmark" by the American Society of Mechanical Engineers.

After WWII, the original jeep continued to serve, in the Korean War and other conflicts, until it was updated in the form of the M38 Willys MC and M38A1 Willys MD (in 1949 and 1952 respectively), and received a complete redesign by Ford in the form of the 1960-introduced M151 jeep. Its influence, however, was much greater than that—manufacturers worldwide began building jeeps and similar designs, either under license or not—at first primarily for military purposes, but later also for the civilian market. Willys turned the MB into the civilian Jeep CJ-2A in 1945, making the world's first mass-produced civilian four-wheel drive. The "Jeep" name was trademarked and grew into a successful and highly valued brand.

The success of the jeep inspired both an entire category of recreational 4WDs and SUVs, making "four-wheel drive" a household term, and numerous incarnations of military light utility vehicles. In 2010, the American Enterprise Institute called the jeep "one of the most influential designs in automotive history." Its "sardine tin on wheels" silhouette and slotted grille made it instantly recognizable, and it has evolved into the currently produced Jeep Wrangler still largely resembling the original jeep design.

By 1940, U.S. policies had caused a stark disadvantage compared to Nazi Germany's aim, building a standard fleet of Wehrmacht (German armed forces) motor vehicles. Since 1933, the German industry could only produce Wehrmacht-approved trucks. The U.S. Quartermaster's only significant success for standardization, through late September 1939 Army Regulations on tactical trucks, was that the War Department limited procurement to just five payload chassis types (categories), from 1⁄2‑ton to 7+1⁄2‑ton—but only "models produced commercially by two or more competing companies..." The Army was still to use "commercially standard" trucks and parts, with only minor modifications, like brush-guards, tow-hooks, etc. Specially designed vehicles or a standardized truck fleet were still ruled out. "This policy was intended to assure speedy production at the outbreak of war, regardless of the maintenance and spare parts problems that might develop later." The new rules more or less allowed the Army to order, in late 1939, the U.S. military's first ever light, quantity-produced 4×4 trucks: the half-ton Dodge G-505 VC-series trucks, delivered in the first half of 1940. Although so well received, that the preproduction order of under 5000 units was followed by nearly 80,000 additional units ordered for 1941, these rigs were still nowhere near small, light and nimble enough for the jobs that both the Infantry and Ordnance branches required it for. By contrast, Germany had already completed a development program to produce off-road capable "Standardized Military Vehicles" (the Einheits-PKW der Wehrmacht), from 1933 to 1938, which had already yielded a fleet of tens of thousands of standardized vehicles for the German Army. Moreover, lessons were learned, and a second program to develop a cheap, light, nimble multipurpose off-roader, the Volkswagen Kübelwagen, had already started in 1938. America's military faced a severe catch-up in time and knowledge. In June 1940, the race was on to produce a lightweight, four-wheel-drive cross-country vehicle for the U.S. Army, capable of carrying equipment and personnel across rough terrain.

The idea of the jeep originated with the infantry, which needed a low-profile, powerful vehicle with four-wheel drive and it was turned over to commercial companies (chiefly Bantam, Willys, and Ford) to deliver—the development repeatedly being described as a "design by committee." In fall 1941, Lt. E.P. Hogan of the U.S. Army Quartermaster Corps wrote: "Credit for the original design of the Army's truck 1⁄4‑ton, 4×4, may not be claimed by any single individual or manufacturer. This vehicle is the result of much research and many tests." Hogan credited both military and civilian engineers, especially those working at the Holabird Quartermaster Depot. Nevertheless, Bantam is credited with inventing the original 1⁄4‑ ton jeep in 1940.