The former first-round pick (No. 9 overall) in the 2011 draft, Hamilton is a restricted free agent and is expecting to earn upward of $7 million per season with his upcoming contract. Because of the Bruins' salary-cap constraints, the team likely wanted the blueliner to accept a bridge deal, but it appears Hamilton wasn't willing to do that and wants a long-term contract. He made $832,500 last season.
"The offer that I offered was certainly in line with comparables," Bruins general manager Don Sweeney said. "So, the spread for me was an indication that maybe we'd be better served if we went in a different direction at this time."
The Bruins selected right wing Zachary Senyshyn of the Sault Ste Marie Greyhounds of the Canadian Hockey League with the No. 15 pick on Friday night.
The Bruins also traded forward Milan Lucic to the Los Angeles Kings on Friday, getting back the No. 13 pick and two players. They used the No. 13 pick on defenseman Jakub Zboril of the Saint John Sea Dogs of the Quebec Major Junior Hockey League. The 14th pick, their own, was used on left wing Jake DeBrusk of the Swift Current Broncos of the Western Hockey League.
Hamilton, 22, played three seasons for the Bruins, appearing in 178 games and recording 22 goals and 61 assists for 83 points, including a plus-23 rating. He played in 19 postseason games and had two goals and eight assists for 10 points.
"We felt that he had already proven that he could play in the NHL and play at a very high level, but at the same time we're always looking for fits and we think that the way Dougie plays the game -- likes to jump into the play, likes to skate -- that's the kind of game that we play," Flames coach Bob Hartley said. "We play that up-tempo game and we think he's going to be a great fit for us. I've just talked with him. I'll talk with him in the next couple of days, also. We'll get him in Calgary for camp. That's where the relationship is going to start. He'll need a little time, but the beauty of the trade is that we have all training camp to work with him, to get him used to how we play, and at the same time to learn a little bit more about him also. He's a great young defenseman, and I think he's going to mature very well with us."
With Hamilton's exit, the Bruins have a huge hole on the blue line, but they did retain a key member by re-signing defenseman Adam McQuaid to a four-year deal worth $11 million, a source told ESPN's Pierre LeBrun.
Information from ESPN.com's Craig Custance was used in this report.