Star Alliance member airlines United and Air Canada attack American Airlines hubs with low fares to Barcelona, Spain BCN from $313 to $406 round trip out of Charlotte CLT, Dallas DFW, and Phoenix PHX.
Flight departure dates are primarily September to December 2017.
Low fare dates are searchable on Google Flights, however, most dates I checked returned fares re-pricing in $800s. Using dates from Google Flights searches on Momondo.com returned ticket prices from $313 for Charlotte CLT to Barcelona and $406 from Phoenix.
Momondo.com search results for United Airlines flights
Charlotte CLT – Washington IAD – Barcelona BCN – IAD – CLT
($405 on Google Flights).
TravelMerry.com online travel agency ticket
Phoenix PHX – Los Angeles LAX – Montreal YUL – Barcelona BCN – Zurich ZRH – San Francisco SFO – PHX
($452 on Google Flights).
Dallas DFW – Toronto YYZ – Barcelona BCN – Toronto YYZ – Dallas DFW
($446 on Google Flights.)