Following the successful fitting of a 105mm howitzer on the medium tank chassis, plans were made in April 1942 to mount a high velocity gun on the medium chassis to provide a complementary SP vehicle for the Tank Destroyer Command. Designated T35 this vehicle utilised an early production M4A2 tank chassis, then just available, with an open-topped low sloped turret adapted from the turret design for the T1 Heavy Tank, and the 3in gun projected for the same vehicle. However, the Tank Destroyer Board asked for a lower silhouette and angled hull superstructure, so an improved design T35E1 was drawn up, again on the M4A2 chassis, and incorporating these features. The T35E1 was modified with thinner armour than the T35 and the circular turret was subsequently abandoned in favour of a five-sided welded turret. As finalised, the design was standardised in June 1942 and designated M10 GMC. In order to increase production, use of the M4A3 chassis was also authorised and vehicles built on this chassis were designated M10A1 GMC. Most of these were retained in America for training or converted to prime movers, M35. Others were allocated to Lend-Lease shipments to Britain. Grand Blanc Arsenal built 4,993 M10s between September 1942 and December 1943. Ford built 1,038 M10A1 between October 1942 and September 1943, and Grand Blanc built 675 M10A1, September-November 1943. 300 of the latter batch, however, were completed with new turrets as M36 (T71) GMC.
M35 Prime Mover towing 240mm gun, France, early 1945.
Full-Track Prime Mover M35: M10A1 converted by removal of turret and fitting of air compressor and cables for towing 155mm and 240mm artillery pieces. Crew: 6; weight: 55,000Ib.
British 17pdr Achilles IIC conversion of M10A1, showing gun at maximum elevation.
A number of M10s and M10A1s were supplied to Britain in 1944 where they were designated “3in SP, Wolverine”. These were issued for combat service to British units in Italy and France; most were converted from late 1944 by replacement of the 3in gun with the British 17pdr gun, producing a much more potent tank destroyer than the M10 in its original form. In its new guise the vehicle was designated “17pdr SP. Achilles Mk IC”. M10A1s similarly converted were designated Achilles Mk IIC. The original mantlet was retained in this conversion. First in service in limited numbers in 21 Army Group in early 1945, the Achilles was used for many years post-war by the British. This was a most successful conversion.
British M10 gun tower converted to experimental mine plough, 1945.
Vehicles not converted were altered to gun towers by the removal of the turret and at least one of these was tested as an experimental mine plough.
Designation: Gun Motor Carriage M10 or M10A1
Crew: 5 (commander, driver, gun crew (3))
Battle weight: 66,000lb
Dimensions: Length 19ft 7in Height 8ft 1tin Width 10ft
Armament: Main: 1 x 3in gun M7 (1 x 17pdr OQF in Achilles) Secondary: 1 x ·50 cal Browning MG (AA)
Armour thickness: Maximum 37mm Minimum 12mm
Traverse: 360°. Elevation limits: +19° to -10°
Engine: Twin GMS 6-71 diesels (M10) Ford GAA V8 petrol (M10A)
Maximum speed: 30mph
Maximum cross-country speed: 20mph (approx)
Suspension type: Vertical volute
Road radius: 200 miles
Fording depth: 3ft Vertical obstacle: 2ft
Trench crossing: 7ft 6in
Ammunition stowage: 54 rounds 3in 300 rounds ·50 cal MG
Special features/remarks: Standard M4 series chassis and motors but with entirely different well-sloped hull affording good armour protection. Retrospective modification was fitting of 2,500lb weight to turret rear to give better balance to turret. Late production vehicles had modified shape to rear of turret. British Achilles conversion with 17pdr gun was much superior in hitting power to original M10.