Historical Information The first KC-135A entered the U. Air Force fleet in 1957; the last was delivered in 1965. About 550 of the 732 tankers built, all by Boeing at its Seattle facilities, remain in service. In addition, Boeing built 88 similar aircraft for other Air Force uses, such as flying command posts, pure transports, electronic reconnaissance and photo mapping. The last of these special-purpose aircraft was delivered in late 1966. Tanker Characteristics The KC-135 is equipped with a flying boom for fuel transfer; a special drogue can be attached to the boom on the ground so it can refuel probe-equipped aircraft. An operator stationed in the rear of the airplane controls the boom. A deck above the fuselage-mounted tanks can hold passengers and cargo.In the past 20 years, refueling requirements of the armed services have more than doubled, making the KC-135 an extremely valuable national resource. Modification programs at Boeing have contributed to the life and performance of the airplane. Those efforts include wing re-skinning and the KC-135R and KC-135E re-engining programs. KC-135 Reskinning Program A major program to replace lower wing surfaces on the aircraft was completed in 1988, with a total of 746 C/KC-135 aircraft " most of them tankers " modified over a 13-year period. The work involved replacing about 1,500 square feet of aluminum of the underside of the wings, which carry most of the wing load in flight, with an improved aluminum alloy. The original wing surface consisted of a type of aluminum more susceptible to fatigue. Skin panels were milled, machined and contoured at Boeing. The wing then went into a rivet assembly jig where stiffeners and skins were joined. The bottom section of the old wing was cut away and replaced by the new sections. In addition to the skin panels, engine strut fitting also were replaced. Each unit required a total of 564 parts, 32,200 steel fasteners and 19,500 aluminum rivets.