I was always under the impression that the B17s were unescorted - which is why they suffered such heavy losses. (forcing the Americans to curtail their daylight raids until the advent of long range fighters like the P47 and P51). Hence, there weren't really any fighters - although Spitfires used to escort the bombers as far as they could (only midway into France). When escort fighters were introduced, the balance of the air war over Germany swung decisively in favour of the Allies in a short period of time.