Boeing wrested a key sale from rivals Airbus Industrie and McDonnell Douglas by winning a $660-million contract from Scandinavian Airlines System for nine twin-engined 767 jets. The announcement by the Stockholm-based carrier ended a year of speculation. SAS also has signed an option to buy 15 more of the extended-range aircraft from Boeing. The carrier said it chose Boeing over its competitors largely because Boeing promised to deliver three of the planes by April, 1989. Scandinavian Airlines is owned by the governments of Norway, Sweden and Denmark.