@Daniel_Cerritos and @Nathan can tell you all about KBOS. But I think JAL flys to KBOS from Haneda. And British Airways does KBOS to EGLL. Cathay Pacific does VHHH to KBOS so the options are looking pretty good.
The longest flight that departs from Boston is KBOS(Boston) to VHHH(Hong Kong). It’s 16 hours long. It’s operated by Cathay Pacific on their 777-300ER.
Although not as long, but close enough we have Hainan Airlines which flies a 787 to Shaghai. Which is a 14hr flight. I’d say it may be a close 2nd place. @Nathan, could you confirm that, please? :)
If you happen to chose to fly with Hainan, just a quick reminder that we currently don’t have the livery nor callsign, so you’d have to use the Generic 787-9 and the callsign: HU7962
Also the flight continues to Chongqing from Shanghai :)