Despite being equipped with the same main gun as some variants of the much larger Sherman tank, the M18 attained a much higher top speed of up to 55 mph (89 km/h) by keeping armor to a minimum, and using the innovative Torqmatic automatic transmission.

After testing, it was revealed that a more powerful engine was necessary, and the 76 mm gun M1A1 being developed for the M4 Sherman had also impressed the Tank Destroyer Force.

The T67 project was closed in January 1943, and the Ordnance Department directed that six pilot models of the 76 mm Gun Motor Carriage T70 be built with torsion bar suspensions, more powerful engines, new turrets and minor changes to the hull front.

Although the Hellcat was "relatively mature" when it came to automotive performance, problems had to be rectified that included issues with the engine and new automatic transmission, weak front shock absorbers, and the position of the gun to provide more working space for the gunner.

Its power came from Wright R-975, a nine-cylinder, 350 to 400 hp (260 to 300 kW) radial aircraft engine, the same as that used on the M4 Sherman tank, paired to a 900T Torqmatic automatic transmission.

Despite its T70 prototypes requiring several improvements, the 704th had a "superlative" testing record, and the unit was later issued production Hellcats after many of their suggestions were integrated into the vehicle.

The breech block of the gun was tilted 45 degrees to the right to aid the loader in manipulating and inserting ammunition in the confined space of the turret.

[4] One disadvantage of the M18 was the inconsistent performance of its 76 mm gun against the thick and steeply sloped frontal armor of later German designs such as the Tiger and Panther.

[25] The 76 mm gun with standard ammunition could penetrate the frontal turret armor of Panther tanks only at very close ranges,[26] whereas the HVAP ammunition gave it a possibility of effectively engaging some of the heavier German tanks and theoretically penetrating the front of the Panther turret at ranges of about 1,000 m (1,100 yd).

In testing of the T70 for possible use as a light tank, 9 out of 30 armor-piercing .30 caliber bullets penetrated the side of the turret at a range of 75 yards (69 m).

In general, Hellcat crews were complimentary of their vehicle's performance and capabilities but did complain that the open top created a cold interior in the Northern European winter of 1944–45.

The rectangular ventilation grate for the transmission and differential oil coolers located behind the driver's hatch initially had an angled cylinder-like shape, protruding above the line of the upper hull.

To solve this problem, a muzzle brake that directed the blast to the sides was standardized in February 1944, but a sufficient number were not produced to allow them to be incorporated onto production lines until the summer.

Beginning in June 1944, roughly the last 700 Hellcats received the M1A2 gun equipped with a muzzle brake, which also incorporated a faster rifling twist.

Beginning with M18 serial number 1701, an engine oil dilution valve and provisions for a battery-powered heating unit that drew fuel from the left fuel tank in order to produce hot air for heating the back of the engine, transmission and differential and their oil coolers, the vehicle battery, and the auxiliary generator in extreme cold conditions was installed.

Later vehicles also featured an M20 azimuth indicator next to the gunner's controls, allowing for easier performance of indirect fire missions.

[45] On 19–20 December, Team Desobry, a battalion-sized tank-infantry task force of the 10th Armored Division was assigned to defend Noville located north-northeast of both Foy and of Bastogne, just 4.36 mi (7.02 km) away.

With just four[8] M18 tank destroyers of the 705th Tank Destroyer Battalion to assist, the paratroopers of 1st Battalion of the 506th Parachute Infantry Regiment attacked units of the 2nd Panzer Division, whose mission was to proceed by secondary roads via Monaville (just northwest of Bastogne) to seize a key highway and capture, among other objectives, fuel dumps—for the lack of which the overall German counter-offensive faltered and failed.

Worried about the threat to their left flank in Bastogne, the Americans organized a major joint arms attack to seize Noville.

Team Desobry's high-speed highway journey to reach the blocking position is one of the few documented cases[8] in which the top speed of the M18 Hellcat, 55 mph (89 km/h), was actually used to get ahead of an enemy force.

A Military Channel historian credited the M18 tank destroyers with 24 kills, including several Tiger tanks, and believes that in part, their ability to "shoot and scoot" at high speed and then reappear elsewhere on the battlefield, confused and slowed the German attack, which finally stalled, leaving the Americans in control of the town overnight.

[8] After the Battle of the Bulge, since numbers of the M36 tank destroyer were slow in arriving to the European Theater and towed tank destroyer battalions equipped with the 3-inch Gun M5 had uniformly performed very poorly in the battle when compared to self-propelled units,[46][47] it was decided to re-equip many of them with the "surplus" of M18s that were then in the theater, with the new M36s mostly re-equipping former M10 units.

"[44] Conversely, the main gun was considered inadequate against the frontal armor of later German tanks like the Tiger and Panther, especially before the introduction of HVAP ammunition.

In addition, even though it could pierce more armor than the 75 mm gun M3 mounted on the M4 Sherman tank, it did not have the high explosive firepower of the former when used for infantry support missions.

Many battalions considered the assistant driver redundant, and removed him, or stated that the vehicle should be equipped with a bow machine gun.

[49] M18s served in tank destroyer battalions and other units supporting US Army infantry divisions in the latter stages of the Pacific war, notably in the Philippines and Okinawa.

Tank destroyer battalions equipped with the M18 that served in the Pacific Theater of Operations included the 637th, 670th, and 671st, although only the 637th saw combat, most notably participating in the Battle of Manila where it earned a Distinguished Unit Citation.

Many intended for European countries under the Mutual Defense Assistance Act were rebuilt and refurbished by Brown & Root in northern Italy in the late 1940s and early 1950s, and bear data plates that indicate those rebuilds.

[54][page needed] The hulls of the M18s were dismantled and the turrets were used as gun emplacements on the northern borders of Greece and the Aegean islands.

640 of the early production M18s that had been returned to the Buick factory for modification were converted into the T41 prime mover configuration between October 1944 and March 1945 by removing the turret and fitting seats for up to eight men in the open fighting space.