A lot depends on what weapons you have, shooting within your convergence and what you hit.
This last bit is the most important one but is not always possible to control to your favor. I have de-winged a He-111 with a half second burst from a Spitfire, using a slightly tweaked belt setup (default plus a bit more AP and incendiary rounds). However i was shooting from quite close at the top of his wing, just outside the engine. The fuel tank blew up and the wing was torn away.
The next bomber i attacked in that mission it took 4 times the ammo to bring down, because i was attacking from the same altitude. I killed the rear gunner at the start of my attack run, so i just parked behind him and kept firing at his engine. I guess most of my rounds were hitting his armor plating.
In short, it's always better to come in from oblique angles to bypass the armor plating, while aiming for the crew (relatively easy with all that glass on LW bombers), engines or fuel tanks. This means you have to be reasonably good at deflection shooting.
With regards to the RAF bombers now, the Wellington was known for its structural strength but at the same time it was prone to catching fire. I think the early models lacked self-sealing fuel tanks, which would explain the vulnerability.
In one of the earliest raids in the war (battle of the Heligoland bight, wikipedia has a pretty good article on it), a formation of Wellingtons on a low altitude daylight raid was all but massacred by 109s and 110s. Certain tactical decisions played a part as well, but the actual vulnerability of the aircraft also became apparent. I think that after that raid it was decided to start fitting them with self-sealing fuel tanks. It also drove the RAF to start considering night bombing and the development of bombsights that didn't require such a long bomb run to aim accurately.
As for the Blenheim, squadrons equipped with it had some of the highest casualty rates in the RAF. Again, choice of tactics played a big part in that, but it too was never known for its durability.
Generally speaking, none of the RAF bombers were (apart from later Wellington versions). The RAF bombers were designed to carry as much as possible, as far as possible and as fast as possible.
This design mentality continued throughout the war, with the Lancaster having one of the worst ratings for crew survivability in emergencies. The competing Halifax was much safer in the event of a crash landing or in-flight emergency. The Mosquito was almost untouchable (unless bounced by single engined fighters that had an altitude advantage) it only carried 2-3 crewmen, was cheaper to built, had a range to reach Berlin, could carry as much as a B-17 and a lot of them had the ability to carry out pin-point strikes against vital targets.
However, most bomber command squadrons were flying the Lancaster, simply because it could carry 8 tons of bombs in that cave of a bomb bay.
The irony is that some historical studies now claim that had the RAF gone for an all-mosquito force, it would have done much more damage to actual military infrastructure with a much lower cost in material and crews.
I hope you find some answers to your questions in this post and excuse me if i'm rambling on a bit. I tend to get carried away because the ETO bomber campaigns are one of my favorite WWII topics
