Best attacking position in any fighter is from directly above. Build up lots of speed, hit them hard and fast, and then get out of their gun range as quickly as possible (which is easy since you have now built up lots of speed in a dive).
Best place to aim is for the wings, specifically between the inboard engine and fuselage. Hit them there and 9 times out of 10 they burst into flames from just a few rounds.
Attacking bombers takes patience. Set up the perfect attack approach to minimize the effectiveness of return fire from the bombers, make sure to land at least some hits to maybe kill some gunners or just damage their plane, and don't loiter inside of 1K of the bombers. Only time you should be within 1k of bombers is when you're making a high speed attack run.
These are important points by Latrobe.
To add.
1. Climb out ahead of the bombers while maintaining 1k distance. This is where patiences comes into play. You want to be out front and above the bombers by 1k before making your attack.
2. Roll over and attack the bombers from a forward top/side angle and drill them from their top 10-11 or 1-2 o'clock position. As Latrobe said either go for the wings, or aim directly on the cockpit.
When you go underneath the bombers pull level and get about 1k away (you will want to be underneath extending away from the right or left side.) Never extend directly behind them. Again patience plays a part here, dont lose your speed.
When you are 1k away pull up above the bombers and again extend until you are above and in front of the bombers. Then roll over rinse and repeat.
I have found this is one of the hardest shots for bombers to make. From the 9-10-11 oclock positions and from the 1-2-3 o'clock positions.
If you are attacking a hoard of bombers. Start from the outside bombers and pick them off either from right to left or left to right. So say I'm going for the guy on the left of the group. I will be 1k away to the left of the entire group and will climb 1k above to be higher than them. When I'm ready. I'll roll over to the right, attack the first left bomber from the top 1-2 o'clock positions and when I go under the bombers roll back slight right (away from the group) and I'll be able to extend the other direction so that I do not go under the entire fleet of bombers. Then I'll pull up after 1k extend climb back to front top position. Rinse and repeat.
Patience and aimimg and position are your biggest challenges and are the most important to attacking those pesky bombers.