Willys MT(-Tug)s alone were built as "Truck, -ton, 6×6, Tractor", under Ordnance production contract W303ORD4623, production order T6620, and even a maintenance supplement for the "6x6 Willys MBTug" was printed with the 1943 TM101513 technical manual. Including miscellaneous test units, a total of 24 units are believed to have been built, with six known survivors. An even smaller number of ton jeeps with a slightly stretched wheelbase were built as the Willys MLW(−1) through MLW-4 "Jungle Jeep". LW stood for Long(er) Wheelbase, to accommodate significantly larger wheels and 7.50–20 tires with a tractor-like profile, with the objective to serve in the jungles of the Pacific theater, after a September 1943 request from the South West Pacific for a truck with payload and mobility over mud and swamps of jungle terrain, superior to that of the regular jeep. Tracked jeeps Several tracked jeep prototypes were built, because of such a need in Alaska and Canada. After America entered the war, a Japanese attack on the Aleutians suddenly made the Alaskan military base a zone of great military importance. The snow-rich circumstances created a need for tracked, jeep-like, all-purpose vehicles, and the Canadian Bombardier company and Willys created the T29 jeep half-track out of one of the existing 6x6 Willys MT chassis. The T-29 'Snow Tractor' (Jan 1943) expanded the rear chassis to a total of six wheels: three on each side, with a broad rubber belt serving as a track, running around two Ford model A wheels, followed by a notably larger wheel at each back corner. Instead of front wheels, the rig got skis, and the front-wheel driveline was omitted, to save cost and weight. It was followed up with the T29E1, on which front wheels returned, but mounted on the front skis, and still non-driven, just so that the front could now both glide and roll. Due to Willys' workload, International Harvester helped assemble a further five T29E1 prototypes. Under the steering front wheels, skis could be mounted or removed. An Aberdeen test report critiqued that the T-29E1 was difficult to steer, as the tracks could not be controlled independently, and that prolonged use caused excessive track component wear. A completely rearranged rear was then proposed, and a T28 litter-carrier was completed for testing by August 1944. The only known surviving half-track World War II jeep is a WillysT28 named 'Penguin'. Further (fully) tracked "jeeps" were also armored, and