Christie continued to modify the M1928 chassis even as it underwent testing. In August 1929 the Infantry Tank Board recommended that the M1928 be included in the army’s tank production program. The chief of infantry recommended that the army purchase five tanks for tactical experimentation and one for mechanical and performance tests. Almost immediately disputes developed between the Tank School, the chief of cavalry, and the Ordnance Department. In March 1929 the Ordnance Department’s technical staff cited problems with Christie’s past work, including poor design and workmanship, and recalled recommendations of 1924 that it terminate all contact with Christie’s projects.
Disappointed with his reception in the United States, Christie opened contacts with foreign governments to sell his tanks abroad, notably to the Soviet Union and Poland. The Red Army acquired several M1928s, and these became the basis for their BT-series tanks. The Soviets’ T-34 tank, which may have been the best all-around single tank of World War II, utilized a Christie suspension system.
Christie’s M1928 weighed some 19,300 pounds, had a three-man crew, and, thanks to its 338-hp engine, was capable of record speeds of 26 mph on tracks and 50 mph on wheels. Its chief drawbacks were its light armor (13mm) and armament (two machine guns). Although the U.S. Army did purchase six M1928s, some criticisms of the tank were certainly justified. The armament was totally inadequate, and the tank itself was mechanically unreliable. It took 30 minutes to discard the tracks in favor of the wheels, and the higher pressure and narrowness of the rubber tires meant that the tank could run only on hard-surface roads in wheel mode. The tracks were also not satisfactory and tended to come off or break.
Christie was well aware of the limitations of the M1928 and soon introduced a new model, the M1931. It was easily the most successful of his tanks in numbers built and influence abroad, with sales to the Soviet Union, Britain, and Poland. The U.S. Army purchased three M1931s, designating it the T3. Christie sold two others to the Soviet Union. The M1931 weighed some 23,500 pounds, had a 338-hp engine, a crew of three men, 16mm armor, and top speeds of 25 mph on tracks and 50 mph on wheels. It mounted a 37mm gun and a .30-caliber machine gun.
Christie next sought to introduce a tank capable of being transported by air. His M1932 made use of lightweight materials. One of the most unusual tanks ever built, it had a forward-facing propeller that enabled it to be dropped from low-flying aircraft, whereupon it could fly down and hit the ground running! Powered by a 750-hp Hispano-Suiza engine, the M1932 could make road speeds of 36 mph on tracks and 65 mph on wheels and could leap a 20-foot gap from a 45-degree ramp. There was thought of giving the M1932 a helicopter rotor, but this was abandoned in favor of attaching conventional airfoils to the hull and working out a system whereby power would be transferred from tracks to propeller at the critical moment and allow the tank to take off. This tank was also sold to the Russians. The M1932 had a crew of three, weighed 11,000 pounds, with maximum 13mm armor, and mounted a 37mm gun and one machine gun.
Christie’s M1936 weighed 13,400 pounds, had a two-man crew, and was capable of cross-country speeds of up to 60 mph. In Britain it evolved into the first cruiser tank. Christie did other defense work as well. He designed the first U.S. standard turret track for battleships as well as gun mounts and carriages.
The T5 Combat Car
While Christie continued to develop original designs, the United States had no armored force, and its tank development was limited to light models. The cavalry and the infantry could not reach agreement on the type of machines required. Finally they settled on an Ordnance Department design along Christie lines, the Combat Car T4E1 of 1931. The cavalry wanted a fast, cross-country vehicle, and the T4E1 met these needs. It weighed some 30,400 pounds, had a powerful 168-hp engine, a crew of four, a top speed of 25 mph, and 15mm armor. Armament consisted of two machine guns. It also incorporated the rear engine and front sprocket drive that became characteristic of subsequent U.S. light tanks.
Meanwhile, other designers were at work. By 1922 Rock Island Arsenal had produced three experimental light tanks: the T2, T2E1, and T2E2. The T2 drew heavily from the Vickers Armstrong 6-Ton, including its leaf-spring suspension system. Because of budget constraints, Rock Island Arsenal came up with a similar vehicle for cavalry use. Known as the T5 Combat Car, it entered service with the cavalry in 1937 as the M1 Combat Car and became the Light Tank M1A1 in July 1940.
The M1A1 light tank weighed some 19,600 pounds and had a four-man crew. Powered by a 250-hp gasoline engine, it was capable of 45 mph. Protected by a maximum of 16mm armor, its armament consisted of three machine guns: one .50-caliber and one .30- caliber in the turret and a .30-caliber in the hull front. The M1A1 had a rugged suspension system, rubber tracks, and a Cletrac transmission to reduce power loss during steering. The improved M2A1 incorporated a trailing idler for better traction and improved ride. The M2A1 had the same armament as the M1; the M2A2 and M2A3 of 1936–1938 had twin turrets (with a 270-degree arc of fire), each mounting a single machine gun.
With the demonstrated combat effectiveness of the full-traverse single turret, Rock Island Arsenal came up with the M2A4 tank. Heavier (23,000 pounds) than its predecessors, it had a single turret mounting a 37mm gun, three hull machine guns, and thicker (maximum of 25mm) armor. Built by American Car & Foundry, the first M2A4s came off the assembly line in April 1940. Experiments were also carried out on some Model M2s with diesel engines and electric transmissions.
The establishment of the Armored Force in 1940 abolished the distinction between infantry and cavalry tank units. The M1 and M2 Combat Cars then became the M1A1 and M1A2 light tanks. Declared obsolete in 1940, they did not see combat in World War II but were utilized extensively as training vehicles and as the basis for subsequent light tank designs. There was also an M2 light tank. It differed from the M1 only in having 25mm armor, a top speed of 45 mph, and three machine guns mounted in twin tandem turrets. When World War II began in Europe the United States had only these inadequate light tanks.
In 1938 the Rock Island Arsenal came up with a design for a medium tank. It did away with Christie’s convertible wheel/narrow track concept. Designated the T5, the new tank was based on the M2 light tank. Its designers sought to make use of as many parts as possible, both for economy and standardization. Thus the T5 employed the M2 radial air-cooled Continental 250-hp engine, transmission, and suspension system. The T5 weighed some 30,400 pounds, had 25mm armor, and was armed with a 37mm main gun in a central turret and six .30-caliber machine guns, sited to provide all-around fire. Tests with the T5 showed it to be underpowered, leading to the substitution of a Wright nine-cylinder, 350-hp radial engine. In June 1939, upon completion of tests, the tank was redesignated the Medium Tank M2. It weighed some 38,000 pounds. Production of 15 M2s at the Rock Island Arsenal began in August 1939, a month before the outbreak of war in Europe.