The current U.S. fleet of aerial tankers consists of a few hundred Cold War era KC-135 aircraft and less then a hundred KC-10 built in the Eighties. The KC-135 is based on the Boeing (BA) 707 airliner but were built as tankers not converted from passenger aircraft. The KC-X program will replace most of the KC-135 if all 400 or more are purchased and built. The initial contract which the proposals are due on is for about 150 aircraft.
Because it will now be several years before the first KC-X flies and a substantial number are available the U.S. Air Force will need to continue to upgrade, maintain and overhaul the existing aircraft. The most modern variant of the KC-135 is the R model. Today ARINC Engineering Services was awarded a contract to upgrade several KC-135R models with new avionics. No value was given for the contract but it will provide modern, digital instruments for the older aircraft. Eventually all KC-135R will receive this upgrade.
Due to the fact that it has taken almost ten years to get to this, the third, attempt to award a contract for a new tanker the KC-135 and KC-10 fleets will fly for several more years beyond what was expected. The continued investment in modernizing these aircraft will help them meet the current set of mission requirements while waiting for the new tankers.