The US M7 Medium Tank was originally designed as an up-gunned replacement for the M3/M5 Stuart Light Tank. But requested changes pushed the design too far increasing its weight to the point where it was reclassified as a medium tank. As a medium tank, its performance and armor protection was inferior to the battle tested M4 Sherman tank that was already in production resulting in the contract being cancelled and later reclassified as obsolete.
During the interwar period, based on WWI experience, the US Army doctrine dictated that tanks were for exploitation, not fighting other tanks but used as a cavalry force to exploit gaps opened in the enemy lines by the infantry and artillery. Separate tank destroyer units were responsible for hunting and destroying enemy tanks. At that time, the US Army preferred light tanks weighing no more than 15 short tons (14 tons) due to their speed and mobility. The US tanks at that time only carried machine gun armament due to a lack of funds and resources. Under the 1920 National Defense Act, only the Infantry was permitted to have tanks, so the new mechanized Cavalry’s tanks were labeled “Combat Cars”. The Infantry was equipped with M2A2 and M2A3 light tanks while the Cavalry units were equipped with the M1 and M1A1 combat cars. In 1940, the M2A4 armed with the 37mm M5 gun came into service and it led to the development of the improved M3 Stuart light tank.
The US Armored Force was created on 10 July 1940 under the command of the mechanized cavalry legend Brigadier General Adna R. Chaffee, Jr. also known as the “Father of the Armored Force”. It took control of all tanks in the US Infantry and Cavalry units being responsible for inspecting, equipping, organizing and training the US Army’s first armored divisions and separate non-divisional tank battalions, including all non-tank personnel assigned. On 15 July 1940, the US 1st and 2nd Armored Divisions were activated, the first two of four armored divisions.
By the autumn of 1940, the US Armored Force began looking at replacing the then in service M2A4 light tank which had fairly thin armor and little potential to be upgraded. Based on the service experience of the M2A4 tank, the Armored Force prepared a list characteristics for a new light tank in January 1941. On February 14, OCM 16582 was issued outlining the characteristics for the new vehicle and the designation light tank T7 was assigned.
Proposed specifications:
– Weight to be 14 tons or less
– Maximum armor thickness of 1.5 inches
– Armament: 37mm M6 gun with coaxial .30 caliber machine gun
– Power operated turret with a gyro-stabilizer
– Two fixed .30 caliber machine guns in the front hull
– A .30 caliber AA machine gun mount on turret roof
– Maintain speed of 12 miles/hour on a 10 percent grade
– Engine capable of starting at -40 degrees Fahrenheit
– Cooling system permit operation up to 125 degrees Fahrenheit
In February 1941, authorization for the construction of two pilot tanks was issued. Various features would be evaluated in these two pilot tanks and, in May, were re-designated as light tanks T7 and T7E1. On 11 July 1941, the Ordnance Subcommittee on Automotive Equipment inspected a wooden mock-up of the T7 at the Rock Island Arsenal located on Arsenal Island (originally known as Rock Island) on the Mississippi River between the cities of Davenport, Iowa, and Rock Island, Illinois, USA. The estimated weight had increased from 14 to 16 tons. Also the low temperature starting requirement has risen from -40 degrees to -20 degrees Fahrenheit and the engine was expected to maintain a speed of 35 miles/hour on a 3 percent grade.
On 1 August 1941, Major General Jacob Loucks Devers became commander of the Armored Force which was headquartered at Fort Knox, Kentucky replacing terminally ill General Chaffee. Chaffee died of cancer on 22 August 1941 at the age of 56, in Boston, Massachusetts. Devers was an articulate proponent of the emerging tactical doctrine of combined arms, rejecting the current US Army doctrine and argued that “the weapon to best the tank is a better tank”. He would press the US manufacturers to produce more powerful engines, and, often against the views of his superiors, pushed the development of more medium tanks armed with a 75mm gun. He also supervised the expansion of the Armor Force from four armored divisions to sixteen.
In August 1941, the program was expanded to include three additional pilot tanks bringing the total to five. The Ordnance Committee Minutes described some proposed features for each of the pilot tanks.
T7 – Welded hull; cast turret; five speed Hydramatic transmission; individually sprung, volute spring suspension; 15 1/8 inch rubber block track.
T7E1 – Riveted hull; formed homogeneous steel plate turret; torque converter; horizontal volute spring suspension; 14 1/4 inch rubber block track.
T7E2 – Cast upper hull; cast turret; Warner Gear torque convertor; Wright R-975 Engine.
T7E3 – Welded hull; welded turret; Detroit Gear automatic transmission; twin Hercules DRXBS diesel engines.
T7E4 – Welded hull; welded turret; twin Cadillac engines; two Hydramatic transmissions and propeller shafts and one auxiliary automatic two speed transfer unit.
On the recommendation of the Assistant Chief of Staff G3, Brigadier General Harry Lewis Twaddle, more modifications to the original characteristics were introduced. The turret side walls were to be sloped at 22.5 degrees from the vertical and a bustle added to the rear of the turret to house a radio. Protective beading was to be applied around the base of the turret. Open sights to be used on the T7E2, T7E3 and T7E4. The +60 and -10 degrees elevation range requirement on the T7 and T7E1 to be reduced on the T7E2, T7E3 and T7E4 to the range normally used against ground targets. It was also proposed to lower the silhouette and provide a better slope on the front armor for the T7E2, T7E3 and T7E4.
In July 1941, Brigadier General Gladeon Marcus Barnes (Chief of Research and Engineering in the Ordnance Department and also designed the US 37mm gun) issued instructions to develop a mount for the British QF 6 pounder Mark III 57mm gun which could be installed in the T7. By January 1942, the major interest was in the T7E2 and it received the 6 pounder as the main armament. The 57mm gun weighed about 784 pounds and fired a 6 pound armor piercing shell at a muzzle velocity of over 2800 feet/second. The hull of the T7E1 was assembled but riveted structures had been shown to be inferior to cast or welded hulls and turrets so work on the T7E1 was abandoned and the T7E1 was used to supply parts for other projects.
During November 1941, the Armored Force representatives inspected a wooden mock-up of a low silhouette version of the new tank at the Rock Island Arsenal. The hull of the new design was 7 inches lower than the original version and the inside width increased from 60 to 75 inches. The Armored Force preferred the low silhouette design and directed that it to be applied to the T7E2, T7E3 and T7E4. The concentration of effort on the T7E2 resulted in slow process on the T7E3 and T7E4. Hercules gasoline engines were substituted for the diesels in the T7E3 and the T7E4 was being assembled with armor plate for ballistic testing. However, the T7E3 and T7E4 projects both were cancelled before they were completed.
This is the soft steel pilot light tank T7 at Rock Island Arsenal on 16 January 1942. It had the same turret used on the M3 Lee medium tank but without the commander’s cupola. The two circular covers on the corners of the hull protected headlights that were built into the hull.
Side view comparison of the T7 (left) and the T7E2 wooden mock-up (right).
Front view comparison of the T7E2 wooden mock-up (left) and the T7 (right).
The T7E2 (57mm gun) arrived at the Aberdeen Proving Ground on 26 May 1942.
The T7E2 seen from above.
Rear views of the T7E2.
The T7E2 was powered by the Wright Whirlwind R-975-EC2 engine which produced 450 gross horsepower at 2400 rpm. The T7E2 weighing almost 26 tons with the high power engine provided satisfactory performance and the suspension was regarded as particularly good. But the Armored Force rejected the 6 pounder (57mm) gun as the main armament. They were pleased with the tank design and its performance but insisted that it was to be armed with the 75mm M3 gun. Aberdeen completed a mock -up of a suitable 75mm gun mount and it was installed inthe T7E2 after firing tests with the 6 pounder. In August 1942, the tank was armed with the 75mm gun and was designated the light tank T7E5. During development, the weight of the T7 increased from 14 tons to almost 27 tons. Due to the heavier weight, OCM 18582, dated 6 August 1942, standardized the light tank T7E5 as the M7 Medium Tank.
Production facilities for the new tank was established by the International Harvester Corporation at the Quad Cities Tank Arsenal, Bettendorf, Iowa. By 5 October 1942, International Harvester had completed three production M7 medium tanks. The first was delivered to Rock Island Arsenal and the second went to Fort Knox, Kentucky and later to Indio, California for hot weather testing. The third tank was shipped to the General Motors Proving Ground in in Milford, Michigan for evaluation. The fourth production M7 was winterized at Rock Island Arsenal for cold weather tests in Canada. Production tanks 5, 6, 7, 9, 10 and 11 were sent to Fort knox for evaluation by the Armored Board. The eighth production M7 was used at Aberdeen Proving Ground for firing tests that could not be done at the General Motors Proving Ground.
During test trials of the production M7s, it was revealed that the M7 was heavier than expected weighing between 28 to 29 tons when fully loaded. That resulted in a loss in performance and production of the M7 was halted while a study was made to reduce the weight. Analysis revealed that some of the weight increase was from the use of castings that exceeded the specified thickness. Orders were given that six of the production M7s to be rebuilt with the lightest possible castings. The power train was also modified to improve performance. Additional modifications were made to the torque converter along with the final drive gear ratio was changed from 2.5:1 to 2.685:1 and the number of teeth on the sprocket was changed from 13 to 14 teeth. Some documents reference the six modified production vehicles as the M7E2 medium tank. Further tests on the modified vehicles showed that the only improvement was in the lower speed range and the performance was considered inferior to the medium tank M4 Sherman then in full production. Production of the M7 was cancelled after only 13 were built. Six of the tanks were the modified vehicles and the remaining 7 were accepted and delivered to the US Army as the M7 medium tank.
This is the third production M7 at the General Motors Proving Ground on 11 January 1943. The USA registration number was “W-3095066”.
These are two other production M7 medium tanks. Both tanks have some of the external storage mounted on the hull and turret.
The machine guns have been installed on this tank. Note the shovel mounted on top of the front left fender. On the lower side of the mantlet just below the main gun barrel is painted “75mm” and note the number 12 on the front left fender.
M7 medium tanks on one of the test grounds.
The International Harvester Bettendorf Works, Bettendorf Iowa, is seen geared up for the production of the M7. These photos were taken on 22 March 1943.
Bay #2 from Column 241 looking East.
Bay #4 from Column 403 – Storage Bay.
Bays #7 and #8 from Column 708 – Tank Storage on Shipping Dock.
A M7 medium tank on a rail car awaiting shipment.
During the development program, an alternate power plant was proposed for the M7 but the weight of the tank would been increased by about 1500 pounds. The procurement of six tanks powered by the alternate engine was proposed in February 1943 and the vehicle was designated the medium tank M7E1. However, with the termination of the M7 production, the project was cancelled prior to the completion of the pilot tank. The projects for the T7E3 and T7E4 were also cancelled at that time. On 20 January 1944, the Ordnance Committee reclassified the M7 medium tank as obsolete.
The US Armored Force finally did received a new light tank mounting a 75mm gun, the M24 light tank Chaffee weighing 20 tons. A new lightweight 75mm gun M6 was developed, a derivative of the T13E1/M5 gun which was mounted in the nose of the B-25G/H Mitchell bombers. The gun had the same ballistics as the heavier 75mm M3 gun used on the Lee/Grant/Sherman tanks but had a thinner walled barrel and a different recoil mechanism. The US Army began receiving the new M24 in November 1944 and they reached the front lines during the Ardennes Campaign in December 1944.
Only one M7 Medium Tank survived the war and became part of the collection at the Aberdeen Proving Ground Museum and was located on Maryland Boulevard, Hartford County, Maryland, USA. It is listed in the collection as “Tank, Light, Experimental, US Army, Steel, Olive Drab, M7, 75mm, US.” Technically, it is a medium tank.
The US Army Ordnance Corps Museum located at Aberdeen Proving Ground was moved to Fort Lee, Virginia (outside Petersburg), as a result of the 2005 Base Realignment and Closure (BRAC) Act. The transfer of the artifacts from Aberdeen to Fort Lee began in August 2009 and was completed near the end of 2011. The museum at Aberdeen was closed in September 2010. Today, the M7 medium tank is stored at the US Army Center for Military History Storage Facility in Anniston, Alabama.
Note the 75mm gun barrel is plugged. The curved metal bracket on the front left fender is a holder for a shovel.
M7 TANK DETAILS
The main armament was the 75mm M3 gun (same gun used on the Lee/Gran/Sherman tanks) with a coaxial .30 caliber on the right side in the combination gun mount M47. The turret was equipped with a powered traverse mechanism and the mount was stabilized in elevation. One .30 caliber machine gun was on the hull bow and another was fitted to an antiaircraft mount on the turret roof. The maximum armor thickness was 2.5 inches and the pistol ports that were on the pilot tanks were eliminated on the Production M7. The driver and the assistant driver were seated in the front hull with duplicate controls allowing either to drive the tank. Above their heads, pivoting hatch covers each containing a rotating periscope for indirect vision and direct vision with armored covers for both crew members located in the front hull armor. An emergency escape hatch was located in the left side wall of the hull behind the driver’s position between the second and third road wheels. The gunner’s position was on the left side of the 75mm gun in front of the tank commander. The loader was located on the left side of the cannon. The gunner had a periscopic sight in the turret roof and an M5A1 telescopic sight on the left side of the M47 mount. The commander and loader each had a rotating periscope in the turret roof in front of two double door hatches. A single non-rotating periscope was located behind the commander’s hatch for viewing towards the rear. A door was installed in the rear wall of the turret bustle could be opened to permit the removal of the 75mm gun. It was necessary to first remove the large external storage box as well as the SCR 508 tank radio and other items from inside of the turret bustle. The production M7 was powered by a Continental R-975 C1 engine (Continental Motors took over the production of the Wright R-975 engine for tank use).
An unique feature of the M7 and all its prototypes was the design which allowed the rapid replacement of the engine and transmission. Both were mounted on rails and could be pulled out to the rear and front of the tank respectively. The objective was to allow the replacement of both components to be completed in 40 minutes. During one practice run, the job was completed in 20 minutes.
Walk Around 1: Prime Portal M7 Light Tank
Walk Around 2: NET-MAQUETTES M7 Light Tank
Note: Although titled the M7 Light Tank these walk around websites are actually of the M7 Medium Tank.
POSSIBLE ALTERNATE HISTORY
If the US Armored Force had approved the T7E2 armed with the 57mm gun and standardized it as the M7 Light Tank, it most likely would have been named the “Chaffee”. The US Army would then had a new modern light tank in service with the potential of being upgraded further. The M3/M5 Stuart tanks would have been phased out of US Army service but still produced for lend lease and supplied to the allied armies. Then the M7A1 would later have been produced armed with the new lightweight 75mm M6 gun and maybe with a commander’s cupola.
MODEL KIT
1/35 Resin Model Kit:
Commander Models 1-029 U.S. M7 Medium Tank
(Requires AFV Club Track #35026)
Website: Commander Models, Inc.
For other scales, the only option is to try scratch building and kit bashing parts from other model kits.
Mike's Research 
Like!! Thank you for publishing this awesome article.
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