Like others have said be patience and setup your attack, DO NOT rush it!
For most bombers high slashing attacks from the 9 O'clock or 3 O'clock position tend to work well as it's generally hard to track fighters from above because of their speed. Also you tend to face less guns.
No matter what happens DO NOT attempt to make a pot shot after your initial attack. Once you've made your pass get away and take your time setting up the next attack. This right here is where I kill most attackers, they setup and make a beautiful attack giving me limited options, however, they screw up and try to get a second pot shot attack in. The problem is at this point they've blown their E and Alt advantage along with the fact that they've now drifted behind my bombers and are easy pickins.
When possible, get a wingman and attack from 2 different directions. Remember a B17 formation may have 30 men in it but they can only look in one direction at a time.
(unless you have a gunner of course)
