Depends entirely on how badly the plane is damaged. If it breaks up in midair your chances are much worse than if it's descending slowly but in more or less one piece and the pilot can keep it fairly steady. Depending on how the plane was damaged you might not be able to make it to your closest escape hatch.

Also relevant is which position you are in. A ball turret gunner on a B-17 or B24 had the worst chances, he wouldn't normally be able to fit his parachute in the turret with him, and if the turret traverse was damaged he would be trapped.

This diagram shows the recommended escape routes for B-17 crewmen. This is for the B-24.

It depends entirely on the nature of the damage. Most bombers were so tough that any damage that would render the aircraft immediately unflyable would entail it breaking up. If this happened, the pieces would tumble, and due to the centrifugal forces and general disorientation that tumbling would create, climbing out of them would be extremely difficult and therefore unlikely. It almost certainly happened many times that a crewman was able to climb out of a tumbling piece of aircraft, but the percentages would be very low.

If the plane was even remotely flyable, chances were pretty good that the aircraft could return to base, or at least get to an area where it would be safe to belly-land, assuming a fighter didn't pick it off first.

The only scenario I can think of where escape would be reasonably possible is if the control surfaces are damaged such that the plane is controllable, but just barely; only enough to keep the plane stable for long enough to bail out of.