At the beginning of the 1939–40 season, Cain was a holdout in the Montreal Canadiens' training camp, bargaining for a better salary. Montreal traded him to the Boston Bruins for Charlie Sands and Ray Getliffe. Cain played his first game for the Bruins, their season opener, in Toronto on November 4, before practicing with his new team. The Bruins lost 5-0 but went on to finish first in the league on the season. That year Milt Schmidt and Woody Dumart led the Bruins goal scoring with 22 goals each, with Cain in third with 21, tied with Bobby Bauer. Cain had seven game winners, and his 21 total goals tied him with Hall of Famer Gordie Drillon for fourth place in goal scoring in the entire league. Only five NHL players scored 20 goals or more, with four of them on the Bruins.
