In the early 1960s, Allis-Chalmers decided to develop a challenger to the John Deere 5010 model. The D21, introduced for sale in 1963, was significantly larger than the brand’s previous models, so both the transmission and the engine were completely new. Likewise, the angular design differed from the rest of the lineup. There were no headlights at the front; instead, they were placed on the rear fenders. The engine was a 7.0-liter inline six, Allis's first direct-injection diesel. It had a power output of 117 hp. The transmission had a total of 8+2 gears across two ranges, and the clutch was 14 inches in diameter. The hydraulics featured draft sensing, but unlike many other large American tractors, it used a gear pump. The steering was hydrostatic; in this respect as well, the D21 was Allis's first. The steering wheel was height-adjustable, with the dashboard adjusting accordingly. There was no differential lock. The 200-liter fuel tank was mounted at the rear of the tractor.

In 1965, the Series II model was introduced, featuring a turbocharged engine with 140 horsepower. Approximately 3,500 D21s were produced, almost all of which remained in the USA.

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