How would a Flying Fortress behave if it lost its empennage mid-flight? The 1990's movie "Memphis Belle" depicts a Flying Fortress having the whole tail cut by a fighter impact on the rear part of fuselage, as a razor or chainsaw will do, the bomber airplane enters a flat descent with all 4 engines operating.

Is this type of accident possible? How would such a bomber behave with the empennage cut? Is there any way or chance to survive such an accident?
Depends on the extent of the damage as well as what systems are affected.  Lose the vertical fin, it's possible the aircraft could make it home but with limited yaw stability.  Lose the vertical fin and the tailplane, the airplane would nose over and depart from controlled flight due to longitudinal instability.