From Battlestar Wiki, the free, open content Battlestar Galactica encyclopedia and episode guide
Sometime ten years prior to Earth's discovery, then-Lieutenant Boomer and Lieutenant Starbuck are on patrol when they are ambushed by Cylon raiders. Starbuck's Viper is severely damaged, and he is forced to put it down on a small planet, stranding him and separating him from the fleet (The Return of Starbuck).
Years later, Boomer has taken the place of Colonel Tigh, and is executive officer of Galactica reports to Commander Adama about the presence of Jamie Hamilton aboard Troy's Viper (Galactica Discovers Earth, Part II). Boomer briefs Adama after Xaviar's attempt to alter history by aiding the Nazis fails, but Xaviar manages to escape. Adama orders Boomer to suspend all other operations on Earth until Xaviar is located (Galactica Discovers Earth, Part III).
Boomer commands the Viper squadron that destroys the Cylons after their attack on the straggling freighter Delphi (The Super Scouts, Part I). Boomer is also a strong defender of the D Squadron, a squadron that leads risky missions against Cylon basestars (Space Croppers).