Trumbo's homer to right-center field with two away in the fourth inning came off right-hander Rubby De La Rosa, the Dodgers' second pitcher of the game.
Trumbo, who has hit 120 homers in six seasons in the Angels' minor league organization, hit 36 homers and drove in 122 runs with Triple-A Salt Lake in 2010. He had one hit in 15 trips to the plate with the Angels in 2010.
"The kid's a strong kid," Angels manager Mike Scioscia said. "He's been swinging the bat well in camp and he got hold of one today."
He could be the Angels' starting first baseman should Kendry Morales' recovery from a broken leg limit him to hitting duties only.
"I think [Bell] feels he has the tools to pitch at a higher level," Scioscia said. "You could see why we're excited about Garrett Richards."
Bell is hoping to make the Angels squad in some fashion. He pitched in 25 games with seven starts for the Angels in 2010, posting a 2-5 record with a 4.72 ERA.
Richards fanned two with a walk.
Dodgers starter Hiroki Kuroda allowed one hit in two innings with a strikeout. Kuroda went 11-13 with a 3.39 ERA in 31 starts for the Dodgers in 2010.
Dodgers right-hander Lance Cormier, a non-roster invitee and a candidate for a starting spot, allowed two hits and a run (earned) in one inning (the fifth).
Matt Kemp, one of three possible starting position players in the Dodgers' lineup, put the Dodgers on the board with his baserunning in the second.
After bouncing a single up the middle, Kemp stole second and took third on a bouncer to short. He then scored on Russell Mitchell's shallow fly to center, barely beating Peter Bourjos' throw to the plate.
Reggie Willits' one-out single to center scored Bourjos from second for the Angels' third run in the fifth.
Chris Pettit added a sacrifice fly in the eighth.