text: The MM-100 used a ,  flat-four engine from the "S" version of the Porsche 356, but this choice proved problematic, as using a foreign-built engine would violate regulations requiring U.S.-built equipment in U.S. military vehicles. Attempts were made to substitute the Lycoming O-145, an American aircraft engine, but it was deemed too expensive and difficult to adapt to a ground vehicle. A solution presented itself when MARCO discovered that American Motors Corporation (AMC) was developing a lightweight, air-cooled ,  V4 engine for a proposed economy car to be sold under AMC's Nash Motors brand. AMC quickly became interested in the MM-100 because it needed a new product to keep a former Hudson commercial vehicle plant operating. By 1954, AMC had completely taken over the project.
  Problems were encountered with the designers' decision to save weight by not using a conventional exhaust system. The prototype did not have a muffler or pipe – rather the exhaust was routed through some frame tubes. This design was inferior because the condensation and acidic fumes caused premature frame failure. There was a competing prototype built by Willys, the 1953 Bobcat or "Aero Jeep", which was to share as many parts as possible with the M38 and M38A1 to save costs. However, the Willys version was rejected in favor of the more advanced M422. In April 1958, AMC and the USMC executed a contract for delivery of 250 production M422 vehicles contingent upon successful trials of the first seven.
  The vehicle was intended to be liftable, underslung the strongest helicopters of the era, the Sikorski H-19 being the benchmark. Although the vehicle was to be used only by the U.S. Marine Corps, it was evident from the beginning that production numbers would remain limited, the vehicle was extensively engineered and incorporated many innovations.