Dylan Strome and Tom Wilson each had a goal and an assist, and Pierre-Luc Dubois had two assists for Washington (2-0-1). Logan Thompson made 20 saves.
David Pastrnak and Morgan Geekie scored for Boston (2-2-0) in its preseason finale. Jeremy Swayman made 32 saves.
The Bruins open the regular season at home against the New York Rangers on Tuesday (8 p.m. ET, ESPN).
Strome scored on the power play at 18:04 of the first period to give Washington a 1-0 lead, putting in a rebound of a Wilson shot.
Wilson made it 2-0 at 1:39 of the second period on the power play when he backhanded the puck into the net after Strome’s pass in front went off the skate of Dubois.
Geekie pulled Boston to within 2-1 at 3:40 of the third period, one-timing a bouncing pass from Pavel Zacha past Thompson.
Pastrnak tied it 2-2 with power-play goal at 5:56, scoring on a one-timer from the left circle.
