I only joined PanAm in DEC 1968, so I missed the DC8 era, but many of my captains had been on the DC8s, and we often talked about their days on that airplane. One thing for sure, the DC8-30s of the 1959-1965 era were all JFK based and operated to Europe. Some were operated towards Panama, Lima and Santiago with the Panagra interchange. The Panagra operation was eventually taken over by Braniff.
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The National DC8-50 flights were primarily MIA-JFK and MIA-LAX flights at the time of the merge in 1980, as well as the DC-10s which were used on flights to LHR. National operations were the start of PanAm's first domestic operations within the USA. But came too late to avoid the eventual demise, as most of the "domestic carriers" were permitted to compete overseas as well.
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Prior to 1980, AA was strictly US domestic carrier (only had YYZ and MEX as "international flights"). As far as United is concerned, their only long range overwater operation was HNL, and no international routes.
Happy contrails