As far as when to retract flaps in a heavy bomber, this is a generic rule of thumb:
Look at your rate of climb before retracting. If you haven't established a positive rate of climb, don't retract any flaps. Look for about 300-500 fpm rate of climb on the vertical speed indicator, then retract the first notch. Once you've established 300-500 fpm using only 1 notch, then you can retract that last notch of flaps.
Also, look at the field elevation. If you're upping heavy Lancs with lots of fuel at a base above 5K, its going to take a lot more runway to take off since the air is less dense, and the engines don't make as much power. I think there are a few MA fields that even go as high as 8K. All the bombers when heavy really take off like pigs when you depart a field that high. The U.S. bombers deal with it better, especially the B-17, which maintains a higher rate of climb than the others as you increase altitude. I think the engines are a bit better with altitude, and the 2000-8000 lb bomb weight difference over the 24 and the Lanc probably make the most difference. Like Schatzi said, fuel has a big weight penalty as well, since 1 gallon of gas weighs 6 pounds. That extra 1000 gallons of gas at 100% fuel equals 6000 pounds.
Sea level bases, I usually don't take off with flaps, but anytime I use a base higher than 4K, I use the flaps. Also, flaps can help you get over some of those low obstacles that seem to always be magically planted right in the departure path.
One technique I use before our squad's bomber night is: I get in a jeep and drive all the runways to see if the departure path is clear. Sometimes, from the tower view, you can't really get a good idea of how the surrounding hills impact the runways.
Good luck
