The way of attacking bombers counts too, you dont want to hang on their 'dead six' (very appropriate) for ages.
Take care of being above them (around 3k) and in front. Remember, do not attack them immediately, first take care of alt, position and speed.
Try and 'fly along' with them a bit at a save distance untill you're ready for your attack.
Dive down, try and be on their side or the front (hitting them from behind never is a good thing)
Make very quick passes, dont spend more than a few seconds firing. If you didn't hit it in one pass, make another, dont take long aim at them.
After this, climb back up to the save distance/alt and regain speed for another run.
You will find out that even .303's can make a difference using this method! (not really, I wouldn't consider attacking a B-17 with .303's but oh well...)