1933-1935 In 1933 a new all-steel covered cab with doors and roll-up windows was introduced. In a 1935 model year style change it had been improved with a "streamlined" V-style windshield. This cab would be used on commercial and military trucks until replaced in 1951. In 1935 the trucks were also improved mechanically and new models were introduced. They developed through the rest of the 1930s. In 1940 Hercules six-cylinder gasoline and diesel engines up to were used and Cummins diesels up to were introduced in 1940.