The D6 started out in 1935 as the RD6, fitted with a 3 cylinder 45 hp D6600 engine. The numbering was changed to the D6 in 1937.Caterpillar first introduced the entirely new D6 in 1941 with the 4R & 5R series. This was powered by the D4600 engine of 55 hp (drawbar).
The D6 4R/5R was replaced by the D6 8U & 9U series, fitted with the 6 cylinder D318 engine of 66 hp, in 1947.
In 1959 the D6 was replaced with the D6B after 60,000 D6 4R/5R and 8U/9U tractors had been built.
In 1963 the D6C was introduced.
In 1977 the D6D with a 140 hp engine was introduced, as well as the D6D SA version with 125 hp (drawbar) for agricultural use.
The D6H was introduced in 1986 along with 3 other H-Series track-type tractors. This was the first D6 with Caterpillar's elevated drive sprocket undercarriage.
The D6R replaced the D6H in 1996.
The D6M was a LGP version with 140 hp.
Since 1996 the D6K at 125 hp and 28,409 lb, the D6N at 150 hp and 36,497 lb and the D6T at 165 hp at 46,690 lb all have been......