The first high-bypass turbofan small enough to use on a narrowbody jetliner was the GE/SNECMA CFM56, which entered service in the late 1970s as an upgrade engine for the KC-135 tanker; Boeing almost immediately began working on a CFM56-powered 707 (the 707 and KC-135 are very close siblings), the 707-700, with a prototype flying in 1979. Boeing offered the 707-700 for sale in 1979 and 1980 (the last 707 version in their product catalog), but got not a single order, and the prototype 707-700 was eventually converted back to a 707-320C and fobbed off on the Moroccan Air Force.
In contrast, when a CFM56-powered version of the DC-8 became available in 1982 (the DC-8-70), the airlines jumped in with both feet, even though the DC-8 had been out of production for a decade (unlike the 707, which was produced all the way up until 1979, and would have lasted longer still had the 707-700 caught on), and would thus be, except for the engines, years behind where the 707-700 would have been.
Why did the CFM56 fare so much better on the DC-8 than on the 707?