The D21 was introduced in 1963, and was the largest of the D series, as well as the first Allis-Chalmers tractor to exceed 100 horsepower and the largest row crop tractor available in the industry; rated at 103 horsepower. The D21 was styled significantly different than the other D series with full fenders, a large platform, and a rear-mounted fuel tank which held 52 US gallons (200 L), allowing an average of 10 hours of field work. The D21 was not turbocharged until 1965, which pushed its horsepower up to about 128 as it remained the largest row crop tractor available. This gave the D21 over 15,000 pounds of pull, making it the largest tractor Allis-Chalmers had ever made, as well as the most expensive at around 10,000 dollars.  

Before this move, the D21 found itself outmoded by the Allis-Chalmers 190-XT, even though that tractor was rated at 93 horsepower. The D21 was a favorite for tractor pullers and still makes good showings at the pulls today.