After the war, Douglas continued to develop new aircraft, including the DC-6 in 1946 and the DC-7 in 1953. The company moved into jet propulsion, producing its first for the military – the conventional F3D Skyknight in 1948 and then the more 'jet age' F4D Skyray in 1951. In 1955, Douglas introduced the first attack jet of the United States Navy with the A4D Skyhawk. Designed to operate from the decks of the World War II s, the Skyhawk was small, reliable, and tough. Variants of it continued in use in the Navy for almost 50 years, finally serving in large numbers in a two-seat version as a jet trainer.