Military trucks and ambulances were built by Dodge, Chrysler, the PT Boat and aircraft engines produced exclusively by Packard, including the V-1650 engine which powered the amazing P-51 Mustang fighter. Studebaker built the US6 truck, the M29 Weasel cargo and other crew carriers, the Journal Standard reports.
Many of the tank retrieval trailers and tank recovery vehicles came from the well known taxi cab factory, Checker. Hudson joined the efforts with its aircraft guns, engines, parts and the “Hudson Invader” engine, known having been used to power landing watercraft on D-Day at Normandy, June 6, 1944.
Many of the military appliances and other specific supplies were assembled by Nash-Kelvinator, while Henry Kaiser became a legend thanks to his California shipyard, where he built the “Liberty” and “Victory” cargo ships in very little time.
From http://www.warhistoryonline.com/war-articles/car-manufacturers-world-war-ii-military-might.html
