The KC-135 was the first offspring of the Dash 80. It was the only jet airplane designed specifically for aerial refueling and for 15 years it was the only tanker used by the Strategic Air Command. More than 600 of the 732 tankers built were still in service in the mid-1990s. Boeing modifications that extend service capabilities of the KC-135s into the next millennium include re-skinning the wings with an improved aluminum alloy and installing new, more powerful and fuel-efficient engines. Two re-engined KC-135Rs can do the work of three KC-135As. The KC-135 remains the Air Force's prime aerial refueling airplane.