During the late 1970s, the USAF opted to commence a series of major upgrades to the C-141 fleet; not only was work started on a life extension programme but, in 1977, the service also accepted a proposal from Lockheed to stretch several aircraft. The first of these stretched airlifters, re-designated "C-141B" to differentiate it from unmodified members of the fleet, was delivered during December 1979. The final C-141B was delivered in 1982. A total of 270 C-141As were modified to the C-141B standard, comprising nearly the entire original production run.