Observing the active French Army involvement in Southeast Asia, the US military began to research a new logistically-minded cargo-hauling vehicle to pair with its new Ford-built M151 MUTT (replacing the post-World War 2/Korean War-era Willys M38 series jeeps). A formal contract offering was announced and, to this, there proved responses from several sources though it ultimately settled to Ling-Temco-Vought (LTV) as the primary designer. LTV unveiled a six-wheeled, 6x6 powered vehicle designed around a primary four-wheeled section attached (by an articulated joint) to a two-wheeled trailer. Despite the joint separating the two sections, the front-most and rear-most axles worked in conjunction, turning in opposite directions to achieve the required turn radius. Pilot vehicles were powered by an air-cooled engine though, after evaluation, this was given up in favor of a Detroit Diesel 3-53 3-cylinder, 2-stroke diesel-fueled engine of 103 horsepower. Overall, the vehicle weighed 7,000lbs, featured a 15-inch ground clearance and could make headway on paved surfaces at 55 miles per hour. Additionally, the vehicle held excellent cross-country performance thanks to its 6x6 powered drive with articulated joint unit and was given an inherent (though limited) amphibious capability.
The "Gamma Goat" - earning its name from designer of the articulated joint, Roger Gamount, coupled with the excellent climbing qualities of a wild goat - entered the US Army inventory in 1969 with deliveries ending in 1973 (under the formal designation of "M561, 6x6, Tactical, 1-1/4ton Truck"). A medical-minded transport variant, the M792, was also developed in time. Production was charged to CONsolidated Diesel Electric Company (CONDEC) of North Carolina.
The M561 proved its worth as a carrier though the design was not without issue. The diesel engine lacked a muffler and its position behind the driver posed problems with the noise being generated. The vehicle operated at such loud levels that ear protection became standard for its drivers. This also led to the vehicle proving something of a tactical liability near an active battlefront - particularly if used as a personnel carrier. The system was generally slow when teamed with fast-moving wheeled convoys and its amphibious capability became something of a moot quality - the Gamma Goat required a rather still body of water to traverse and this at a top speed of just 2.5 miles per hour (movement managed solely by the action of its spinning wheels). Mechanics were not fond of the vehicle's heard-to-reach sections which added to maintenance turnaround times and steering the 6x6 machine, while excellent when going off-road, proved something of a challenge to the inexperienced when on stable, flat finished roads.
Gamma Goats were produced to the tune of 15,274 examples before their manufacture run had ended. They served in the US Army inventory throughout the remainder of the Vietnam War before they were given up for good with the arrival of cheaper utility trucks and the ubiquitous HUMVEE multirole vehicle. Over its service life, the Gamma Goat was modified from its base personnel / cargo hauler form into specialized mortar, artillery spotting, radar, ambulance and communications vehicles. The M561 only entered into foreign service with the Mexican Army and Navy.