Fly nonstop on Iberia Airlines from Boston to Madrid $502 round-trip, March through mid-June, then again from mid-August onward. Unfortunately no peak Summer July dates are available at this price point.
Seat inventory is wide-open, travel any day of the week with only a 7-day minimum required stay.
Searching on Iberia.com we found seats departing BOS on Monday, August 13, and returning from MAD on Sunday, August 26, 2018. Travel to Europe in August can be very expensive so we would recommend you purchasing this deal while it lasts.
Take a peek at our Boston BOS fare listings page for a complete look at both domestic and international fare finds.
If you need a hotel while in Madrid, here's whats available for the above dates on Hotelwatchdog.