American Airlines have put many USA to Europe routes on sale.
For example, fly from Boston to Zurich, Switzerland for only $317 roundtrip with American Airlines (see image below).
Boston/Los Angeles/New York/Miami/Chicago/Dallas/San Francisco/Baltimore, USA
(same as DEPART)
Availability from January to March 2019
Allow the search to complete, then click an OTA to view the advertised price: