Some bomber pilots are much tougher because they're more disciplined with the guns and know how to lead better. If the guy opens up on me at 1.5K as I run in at 450 IAS, I know I'm in the chute, and will whack him. If I'm closing to 800 before he opens up, I know I'll have a hard time. To avoid the guns, you want maximum rate of closure between you and the bomber. A head on attack is ideal here, but limits your firing time and increases the chance you'll collide. If you're in a heavy cannon plane, no problem. Open up much earlier because with a lot of closure, you can shoot at longer range. For me, I'm almost always in a Jug, and don't have the cannon. In most cases, you'll find me tipping in from just above. I try to line the bombers up, then let them pass under my nose, out of sight. This allows the engine to protect you from pilot wounds. I try to anticipate where they are and begin pushing the nose down at around 1.0K and open up, leading the nose of the bomber with my rounds. I continue to lead the nose the entire attack, and dive underneath without ever leveling out behind them. Use small pitch changes to walk the rounds into the part of the plane you want to hit, or maybe a little rudder against Lancs. If you do this, you'll almost always lay a lot of rounds on them without risk of collision. After this pass, extend away like Bozon said, using my speed to get about 1K in front of them and then pull up hard and zoom climb to about 2.5 to 3K above them (icon range). This pass and zoom should put you a bit in front of them and high. I always attack the left drone first. Against B-24's, always aim for the wing root. They catch on fire almost as easily as Zero's. Against Lancs I always aim for the outboard wingtip. Against B-17's, I always aim for the tail assembly. B-17's are the toughest to attack without a doubt, as they absorb damage better than any other. If I do get hit, typically its an oil hit and maybe a gun. Plan on getting dinged, especially against the better pilots, just make sure you manage where you take damage. I can always fly away and make the closest base with an oil hit. Just don't linger on their 6 o'clock or they will tear you up. Speed is the key. If you're not getting close to compressing, you're not going fast enough. Use WEP in the dive to accelerate as fast as possible. It takes some practice learning how you want to set up your passes. Film your attacks to determine what you did right or wrong. Then go up and try it again. The good news is that there are always bomber pilots that do not take advantage of altitude and will drone into a base at 10-15K, so you will always be able to find opportunities to practice. When I first started, I used to take a P-47N with max fuel and drop tanks, climb up to 20-25K or so and just drone around looking for bombers to attack. Watch the DarBar and see if you can see guys taking off 3-4 sectors behind the fight and put yourself in position to pick them up.
Good Luck. I always like to see people attack bombers well. My personal philosophy is that unescorted bombers in this game are lambs for the slaughter, if you're patient, smart, and follow some of the advice posted in this thread.
