Southwest Airlines says it will fly to Baltimore-Washington International Airport when it begins service next spring out of Portland. The airline says it will initially offer nonstop flights each day between Portland International Jetport and Baltimore-Washington when it starts flying out of Portland on April 14. There’ll be three flights going each way on weekdays, and two each on weekend days. All flights will be on 143-seat Boeing 737s. The airline’s arrival in Maine has been anticipated since it bought AirTran Airways last year. Southwest has been transitioning its AirTran cities to Southwest and eliminating AirTran service where the two carriers duplicate service.