The new service connects O'Hare International Airport and Athens International Airport in Athens, Greece.
Boeing 787-8's heading to and from Athens will be operating daily during summer, until Aug. 26.
With the addition of nonstop flights to Athens, United's summer schedule out of Chicago is 10% larger than last year, the airline said in a news release.
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United Airlines offers more nonstop, transatlantic flights from Chicago than any other United States airline, the release said.
United offers nonstop flights out of O'Hare to at least 15 cities in Europe, including Amsterdam; Brussels; Edinburgh; Frankfurt; Reykjavik; Munich; Milan; Shannon, Ireland; and Zurich.