In operation since 1957, the KC-135 Stratotanker is used primarily to re-fuel aircraft during flight. 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.