On the nights of June 16-17, 1775, the Americans fortified Breed and Bunker Hills which overlooked Boston Harbor. Although they had not officially declared their independence, a fight for control of the hills became necessary.
When the British advanced up the slope the next day, according to legend they saw a red flag, but we have no real knowledge of which American Flag was actually flown in this battle.
But John Trumbull, whose paintings of Revolutionary War scenes are quite famous, talked to eye-witnesses and his subsequent painting depicting the battle displayed the Continental flag as shown here.
Many historians think the flag more likely to have been at the battle, if any, was the more common First New England Naval Ensign.