This is a common problem with new players in this game.
They are used to more 'gaming' type simulators that let you take off and do wild maneuvers right off the bat. Or perhaps they are more used to jet-fighter games that are a lot more maneuverable during take off than these old WWII jobs.
You have to keep in mind this game is more of a simulator than a game. It is designed to be more realistic when it comes to physics and plane handling characteristics. As a result when you are 'low and slow', like during takeoff, you have to be very light and easy on the stick. Otherwise, the plane will stall (by stalling I mean losing lift, not the engine stalling out) and you will crash.
Three tips:
1> Take only as much fuel as you need. More fuel equals more weight, which means you need more speed and have less maneuverability.. ESPECIALLY with bombers. Most big bombers, such as the lancasters and b24's, can go just about anywhere on the maps with only 25% fuel.
2> Build plenty of speed.. if you are too slow and try to pull up, you will stall. Larger bombers often need most of the runway to get up to speed and take off.
3> Be easy on the stick. Use slight gradual movements. There is rarely any need to whip the stick all the way one way or another. All this does is lead to stalling and loss of control.