Wing fuel tanks - few well placed rounds light them up good.
Though the usual frontal approach works very well, too and B-17s are slow so to set up the head-ons you don't need too much time - and in a Fw190 you can bring them down with even a not so well placed burst.
What I like to do - besides coming in from dead ahead, is getting well above and somewhat in front and to the right/left of the bomber, then dive down - my wings nearly parallel to the bombers direction of travel, and correct my approach with rudder. Shoot and exit safely behind bomber. Most of the time worth the effort - next to zilch chance to get hit by defensive fire. Additionally, the bomber flies through your bullet stream, and if you can get your aim lined up with targets cockpit/tanks (which by the way are bigger targets from above) you don't need to be spot on with timing.

S!

I'm a mostly 109 pilot, but I think this tactic can be used in any fighter against bombers:

If possible, I will make an initial head-on attack targeting the cockpit. If the pilot isn't dead yet, I will proceed to climb over (and even a bit ahead of) the bomber from where I shall execute a 90 degree dive towards the bomber. When the bomber appears from under my nose, I start to lead him and open fire with MGs (which there are almost unlimited supply on the 109 :) ). When the MGs start to hit, I open with cannon. I get a good 3-5s of cannon time on the target with high deflection and exit the attack usually behind and under the target with high speed and pull up again infront of it. Rinse and repeat if necessary. Usually I don't get any hits at all on my plane, and usually the bomber ends up destroyed after the first overhead attack.

Nice tip, I will try this.