I have not flown with the Boulevard, but I did fly from Chicago to Taiwan and Japan on United with a Britax Marathon. It fit just fine into the airplane seat. My almost two year old daughter was able to sleep for much of the flight in her car seat.
It was a big pain in the butt having to haul that car seat around at the airport, trying to board and disembark, and having it take up space in our tiny hotel room in Tokyo. I also bought one of those Travel Go-Go things to help wheel it around.
Good luck to you! I think your car seat will fit in the airplane seat. Hopefully, your daughter will be able to sleep on the LONG flight.