I believe that the B17s had many oxygen tanks. Because the plane was never pressurized, everyone on board had to use the oxygen masks once at a certain altitude. Each member of the crew had his/her oxygen mask with them including a few oxygen tanks. That is including the gunners, pilots, navigators, and other crew members aboard. Majority of the time, there was enough oxygen to last the crew a full flight. Thats not to say there have been malfunctions because you must remember that this is back in the 1940s. Technology wasnt the brightest...
The B29 was the first pressurized bomber in WW2. The use of oxygen masks was minimal with the exceptions of emergencies.