These tractors featured three-point hitches, power steering and diesel engine options.

The engine was coupled to a four-speed transmission with a transfer case, giving a total of eight forward gears and two reverse.

High-clearance and rice cultivation models were produced, and orchard shield kits were available.

The early D15 was essentially the same as the D14 with the same engine but a higher compression ratio and increased RPMs.

The diesel D17 had a six-cylinder engine with 262 cubic inches (4,290 cc) and weighed about 4,660 pounds (2,110 kg).

The diesel model was the first in the industry to come standard with a turbocharger, boosting both horsepower and fuel economy.

[13] 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 (77 kW) with a 7.0 litres (430 cu in) engine.

[14] 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 hp (95 kW) as it remained the largest row crop tractor available.

This gave the D21 over 15,000 pounds (6,800 kg) of pull, making it the largest tractor Allis-Chalmers had ever made, as well as the most expensive at around 10,000 dollars.

[13] Before this move to turbo, the D21 series I found itself outmoded by the Allis-Chalmers 190XT, which could be turned up to produce more power than the D21.

The D21 was replaced in 1970 by the Model 210 and Two-Twenty, also with a 7 litres (427 cu in) engine, which was rated at 122.4 and 135.95 horsepower (91.27 and 101.38 kW) respectively.

The D21 was a favorite for tractor pullers and still makes good showings at the pulls today.