Komatsu exhibited a prototype model of its 120T bulldozer, D555A, at the CONEXPO’81 in 1981. The D555A is capable of mass earth-moving with high productivity - a requirement that is growing rapidly, in recent years. Together with innovative features which provide for high machine availability, high durability, and, operational ease and low fatigue, the D555A is designed to be the forerunner of next generation bulldozers. At present the D555A is being field-tested, and will be placed on the market in the near future. Here we shall present the design concepts underlying the giant D555A, and the steps taken in achieving the essential design targets.