The KC-135 Stratotanker remains the backbone of the United States Air Force aerial refueling capability (with around 376 remaining in service). However, the KC-135 is aging (it was replaced by the Boeing 707 airliner and has been in United States Air Force service since 1957). It is currently being partially replaced by the KC-46 Pegasus but may also be eventually replaced by future platforms like the Next Generation Air-Refueling System (NGAS). There is no one replacement for the KC-135.