Hrm... this is odd... I had a chance to whip up on Author65 (he just keeps reupping bombers for the same field over and over and over, usually milk running so gives you no opposition if you want to shoot him down) so I took up 109s with 30mm, and fired just the 30mm guns.
B26s -- tough, but land hits in the same area and you see damage
B-24s -- not as tough as B26s, easily damaged with 30mm
Lancaster -- head on, fired 12+ 30mm rounds directly into its nose, forward belly and 1 wing root and no damage. Not even an oil or gas leak, nothing, nada zip.
Got all of these on film, too.
Reviewed the lanc film to see exactly how bad I missed -- I didn't miss at all. I pounded him like a meat tenderizer does meat, and he didn't take any damage. A 190a8 finished him off and I got the kill (meaning the server recognized my hits, they were valid hits, but it also means my hits didn't do any damage to a plane they should have incinerated).
EDIT: actually I reviewed some more film.. THe b26s are too tough... It takes upwards of 10 rounds landing to do anything. I made a high speed pass across one's 2-3 oclock and landed a dozen hit sprites, and I think the only damage he got was an oil leak. I ended up finishing him off in another pass, but he should have popped instantly in the first.
The b24s took no damage other than fireballs, and I let them burn up instead of wasting ammo, aside from that they took quite a few rounds wihtout losing ailerons, flaps, or anything. Just caught on fire, nothing else.
EDIT 2: more review of same films... B26s are either experiencing the lancaster problem or are modeled as the toughest plane in the game. While not as pronounced as the lancaster, it takes a lot of 30mm before showing any signs of damage. The B24s I mentioned: one of them DID get a gas leak in the outer wing, as well as get set on fire, but the other did not. I popped the lead in a HO with lots of hits, but can't count that due to instant pilot kill.