The KC-135 was the only jet airplane designed specifically for aerial refueling. More than 732 KC-135s were built, and most, with modifications, remain in service into the 21st century. The KC-135 shared the Dash-80 prototype with the Boeing 707 commercial transport, but was launched first in 1956. The KC-135's aerial refueling equipment in the lower deck leave the upper deck clear for 25 tons of cargo or 80 troops. KC-135s supplied warplanes with fuel during the Persian Gulf conflict, Operation Enduring Freedom over Afghanistan, and for homeland defense.