Staff Writer (Updated: 9/16/2016):
The original M10 became the definitive American tank destroyer of World War 2 and was produced in nearly 7,000 examples for use by the US Army as well as the armies of its allies. However, the type was based upon the modified chassis of the M4 Sherman and lacked much in the way of armor protection. Its main gun was eventually shown to be largely inadequate as the war progressed. The M36 "Slugger" was then developed from the M10 family lineage to field a more potent 90mm main gun and this design was also based on the classic M4 Sherman tank chassis. It saw production reach 1,400 examples and was capable of tackling the latest German tank offerings but appeared towards the end of the war.
The design that was to become the M18 Hellcat was already in development as early as December of 1941. The Japanese attack on Pearl Harbor that month ushered the United States into total war with the Empire of Japan to which Germany and Italy declared war on the United States. British PM Churchill convinced US President Roosevelt that tackling Hitler and Mussolini in Europe and Africa was of priority so resources were geared towards such a foray.
A US Ordnance Department specification was issued for a "fast tank destroyer" design centered around the fitting of a 37mm main gun mount. The vehicle was to be powered by a Wright Continental R-975 C-series 9-cylinder radial engine with the suspension system based on the famed Christie arrangement. This made the new tank destroyer design the first American vehicle specifically developed for the tank-killing role as opposed to a design being a modification of an existing hull to fit the role.
By the middle of 1942, the T49 GMC ("Gun Motor Carriage") prototype emerged for review. By this time, actions in North Africa had already shown that the originally-intended 37mm armament was vastly inferior for the tank-killing role considering the armor quality of the latest German tanks. The prototype was given a 57mm caliber armament instead and was now fitted with a more universal torsion bar suspension system. Evaluations were ongoing and soon the need for ever-more potent armament became apparent for the German Army's reach had now gone beyond the point of fielding Panzer III and Panzer IV medium tanks - the excellent Panther and Tiger I heavy tanks were soon forming the backbone of all German Army actions and proving quite the fore. These German tanks sported highly-capable main guns with thick armor arrangements that outclassed all Allied tanks of the time. As such, the 75mm M3 main gun was selected for the future development in America's new tank destroyer.
The changes soon produced the "T67 GMC" pilot vehicle design which was a lighter, smaller vehicle the 75mm M3 main gun. The idea of fitting a 76.2mm main gun created the "T70 GMC" prototype model which became the preproduction design for what was to be designated the "M18 GMC". Serial production began in July of 1943 and spanned until October of 1944 to which 2,507 examples rolled out of American factories and into war. Operational status was achieved in the summer of 1944 and the M18 received the unofficial nickname of "Hellcat" in due time.
In the field, the M18 proved an excellent vehicle for the intended role. Her power-to-weight ratio provided the vehicle with an excellent top speed of 55 to 60 miles per hour in ideal conditions - faster than any other armored fighting vehicle of the war. The Continental powerplant allowed for great acceleration while the coupling of the engine with the chosen hull design made for a rather agile mount. It was the vehicle's speed that was often utilized as an inherent defensive measure where her armor protection could prove suspect, allowing M18 crews the capability to fire at an enemy and then retreat at speed before a response could be mustered. Her powertrain was noted as reliable and a major asset considering the unforgiving nature of war. The M18 was much smaller in profile and size while being half the weight of the M10 series before it. This made her a harder target to spot and engage. Her armor was improved over that of the M10 but still not entirely adequate when facing off squarely with enemy combat tanks - this job better left to the M4 Shermans and hidden anti-tank gun crews. German tankers actually enjoyed a major advantage when able to target the thinly-armored M18s. The gunnery crew still operated within an open-topped turret emplacement. While this exposed them to unnecessary battlefield dangers and the elements, it provided for unfettered views of the action and plenty of headroom for which to manage the functions of the main gun in the heat of battle. The 360-traverse of the turret also allowed the M18 the ability to engage targets in any direction without the need to turn the entire vehicle against a danger. In contrast, other tank destroyer contemporaries fielded in the war had their guns set within fixed superstructures - necessitating that the entire vehicle be pointed at the enemy. The M18 was nothing short of versatile, robust and effective throughout the war, in many ways making her the finest American tank destroyer of the conflict and a proven war-winner.
Combat actions involving the M18 found her battling across the European countryside and through its many villages en route to the German border. The new German Panther and Tiger I tanks showcased enviable armor protection and excellent gun arrangements but what they lacked was agility - itself a strongpoint of the M18. The M18 could maneuver and engage the weaker side and rear armor facings (through "flanking" maneuvers) typically before the German tanker crews could turn their turrets to counter. This helped M18 crews attain excellent initiative and limit their own losses in the field to a certain extent. The 76mm main gun on M18s was not entirely consistent but got the job done. The only German tank target the M18 crew could not match pound-for-pound was the heavier Tiger II series - but few Allied tanks could stand up to this beast and a lack of fuel and resources across wartime Germany was its biggest threat.