The American rule of thumb was damage bad enough to keep the plane from maintaining position in the formation, or making it unlikely the plane could make the full round trip meant turning back. Breaking formation over Germany was almost certain suicide, because it meant any German fighter who saw the bomber would be able to line up and make an unimpeded strafing run with only one or two guns bearing on him (from bad angles). The defense bombers had before long range fighter escorts wasn't just self defense, it was the overlapping fields of fire formation flying allowed. German pilots did not like flying into one of these formations at all, but one bomber flying alone was easy prey because the Luftwaffe had developed tactics for taking on a bomber from angles where few of the guns could bear, and at a bad angle for the defense gunner to sight and shoot on. There was some discussion on that here: http://www.reddit.com/r/AskHistorians/comments/1vlj4a/in_wwii_bombers_were_wellarmed_with_turrets_why/

Ultimately the decision to turn back or not was the pilots, and the pilots (and everyone else serving) were highly motivated to complete missions if it was at all possible. A crew member being injured might be a reason to turn back, but the likelihood is this would happen so close to the target that the bomber and the rest of the crew would be better served by staying in formation, even if it took longer to return to base. For damage of any kind, either the plane could stay in formation or not. If the plane could keep it's place in formation the pilot would stay with the group. For bombers, safety (what little of it there was) was in maintaining formation. Early in the war to get overlapping fire, later because the escort fighters were staying with the formations, not escorting individual bombers.

As far as sources go, besides the ones in the link there are just so many it's hard to even know where to start. I've read enough unit histories and individual biographies that this answer seems a little like a general knowledge question, at least to someone with an interest in the period. I could probably find a couple of decent items though, if you want to follow up more.

That's a very good question, but i'm a little confused on the part about wounded crew on the outbound flight. We'll get to that part in minute though. Also, the word outbound can either mean outbound from the home airfield to the target (which is what i'm assuming you mean) or outbound from the target area back towards friendly territory.

Let's look at a few scenarios:

So let's say a bomber formation takes off from somewhere in England, and it can be either British Lancasters or American B-17's. This is where the wounded crew part wouldn't really make sense, but if for some reason if a crew-member became seriously ill while over friendly territory, then the plane would probably turn back or divert to the closest airfield to get the guy some medical attention.

A more likely reason for an aircraft to turn back is due to engine problems. For the American's and the British forces, this was a common occurrence from 1943 onwards. In fact, Bomber Command became so concerned with the amount of returning aircraft that claimed "engine trouble" that the USAAF Office of Statistical Control (of which Robert McNamara was a part of) was tasked with finding out the veracity of these "faulty engine" claims. If you'd like, you can view the raw data of their findings [here] (http://www.usaaf.net/digest/t119.htm).

On the Eastern Front however, claims of engine trouble plagued the Russian and Luftwaffe forces on the Eastern Front for the entirety of the war. From the Russian standpoint, the Soviet Union lacked a reliable engine that could sustain long range bombers (the AM-34 and AM-35 were notorious for their poor quality and for being under powered) and focus was shifted away from long range tactical bombing to close air support. Thus, the IL-2 Sturmovik became the bomber of choice for the Soviet Union, while the Stuka was the notorious German counterpart. This actually decreased the susceptibility of the bomber aircraft towards turning back, because their targets were relatively close, but this seems like more of a historical side note in regards to your question. Also, weather was always a factor. This was especially true on the Eastern Front, where it would get so cold that engines would freeze solid (due to the increased viscosity of the oil.) In all Theaters of Operations, however, bombing missions were often delayed or diverted due to adverse weather conditions over the target area.

Now on to your next question: was leaving the formation a big risk? Absolutely. The box formation was proposed by General LeMay in 1942, and was found to be one of the most effective ways in concentrating the firepower of the individual planes, while also raising the morale of the bomber crew due to the "safety in numbers" mentality. Thus, leaving the box was avoided at all costs. If a bomber were damaged by AA or enemy aircraft, then the pilot would try to stay with the formation as long as possible. If the wounded bomber were to leave the formation, the next best tactic is to stay in the clouds and run for home, because altitude at this point doesn't matter.

So let's assume we have a wounded bomber that has turned for home, is limping on 3 engines, has some wounded crew members, and has jettisoned it's bomb load, along with anything else that isn't deemed necessary (in order to conserve weight, fuel, and increase speed.) As she runs for home, dipping in and out of the clouds to hide herself from preying enemy fighters and AA batteries, the next big factor is: location, location, location. After the Normandy landings, the Allied front-line began to push farther East towards Germany. If this wounded bomber is limping home after June 6th, 1944, her crew have a pretty good chance of survival due to 1. Allied air superiority and 2. Friendly front lines that allow for the crew to bail out over. If it's not yet June 6th, then she has to make it over the English Channel before her crew can bail out. I mentioned that some of her crew are wounded; if they are wounded to the point that their survival is in question, sometimes the injured crewman was ejected from the plane with the intent of giving them a chance of receiving medical attention by either their German captors or friendly French forces.