JetBlue Airways is pushing its fleet of single-aisle planes deeper into Europe next year with seasonal daily flights to Milan, Italy, and Barcelona, Spain, from Boston, its latest bid to win over higher-spending vacationers.Daily service from Boston to Barcelona is set to debut on April 16, and the daily Milan flight from Boston is scheduled to begin May 11, the airline said Wednesday.U.S. carriers — from JetBlue's new earn-and-burn miles partner United Airlines to smaller carriers like Alaska Airlines — ha ...