Until 1945, the USAAF bravely bombed Axis targets in daylight, often unescorted. The Schweinfurt - Regensburg raid is infamous in August 1943 for the heavy losses inflicted on the B17's. 60 aircraft were los as were a total of 55 crews taken prisoner, interned in Switzerland or killed. http://militaryhistory.about.com/od/aerialcampaigns/p/regensburg.htm
The plan was to have 2 bomber groups with 10 minutes between them taking off from bases in Britain. The first group would meet expected Luftwaffe fighters on the Regensburg raid, the target there was the Messerschmidt factory. Wile these fighters were refuelling the second wave would bomb the ball bearing factories at Schweinfurt. Both USAAF groups would land in North Africa. http://www.historyandheadlines.com/august-17-1943-us-8th-air-force-loses-60-b-17-bombers-raid-schweinfurt-regensburg/
Unfortunately, bad weather meant the second wave was delayed, the Luftwaffe had time to re-arm and refuel,the disastrous day for the USAAF became known as 'Black Thursday'. The development of the P-51 and Lightning fighters, meant that by March 1944, Allied bombers had protection to and from the target in Germany.http://www.nationalmuseum.af.mil/factsheets/factsheet.asp?id=513