Rheinmetall-Borsig started to develop the Mk 108 as a private venture in 1940 and submitted to the RLM in 1942 in response to a requirement set by the RLM for a heavy weapon mounted on aircraft for use against Allied heavy bombers which were appearing in large numbers by then over German controlled regions. It wasn't submitted as a response to the increasing ruggedness of Allied fighters like you claimed.
Testing of the MK 108 showed that it only took an average of 4 30mm rounds to shoot down a B-17 or B-24, compared the average of 25 rounds from a 20mm MG 151/120 cannon.
ack-ack
Way to copy and past wiki, there, ack-ack...
Yes, I am aware of the testing and rounds required to take down bombers. I have also read a number of references and citations over the decades I've been reading up on the subject that directly attribute the need with the rising loss of expert fighter pilots and the rising number of completely unskilled replacement pilots.
I could draw a parallel to the Soviets and their very untrained pilots in the first half of the war -- despite relying on an ariframe that carried one of the worst 37mm guns of its time, they racked up many kills in P-39s, and later in Yaks with 37mm also. Soviet pilot training was slow and lethargic and wholly unimpressive. By the final days of the war they were putting out fresh pilots trained as well as any other nation, but up until then it was woefully inadequate. Yet with a weapon that killed in less shots those unskilled pilots were able to kill and move on to other targets much faster.
So, yes, while it was beneficial to taking out US heavy bombers, many members of the RLM felt that there was no need for such a weapon against heavy bombers until US heavy bombers were encountered in 1943. Well AFTER the initial development began on the gun. This RLM denial of the issue at hand is one reason for the sluggish progress on the Mk108, even though line pilots were starting to feel the hit.
They couldn't have known how hard US bombers were to shoot down until they actually encountered them. RAF Bomber Command was almost entirely running night bombing at the time. Daylight fighters weren't being used to hunt down RAF heavies. That was the job of the night fighters (Ju88s and 110s). Galland up-gunned his 109Fs, if you recall, adding MG/FFm outboard and upgunning the 7mms to 13mms. This was before US heavy bombers were even a threat. 109F-2s were upgunned from the flat-trajectory MG151 15mm cannons to MG151/20 20mm because there just wasn't enough firepower. 109Gs were being geared for heavier weapons loads (multiple gunpods). All of this is before any US bombers were even on the scene.
Besides, if I recall, the Mk108 met the requirements of an order that said the RLM wanted a weapon that could be fired from outside bomber defenses... Not that it was to take down the bomber in 5 hits or so. The Mk108 met this requirement, but it was already being developed anyways. It was a scaled-up version of the MG/FF cannon. It just so happened to meet one of the RLM requirements so it was given specific attention.
