If they were not pinned in place by g-forces and had a parachute on and could reach an escape hatch, open it and manage to get through it, yes they could, although what awaited them on the ground who knew.

It wasn't always easy though - the rear gunner position in the Lancaster was very tight, so tight that one couldn't wear a parachute and still fit, so your parachute hung nearby, if the Pilot gave the command to bail out or to repair to abandon the aircraft you'd have to unplug your electric flying suit & radio comms, get out of the turret, put your parachute on, plug the suit & radio back in and then squeeze back in to the seat.

You can see the parachute hanging up in this picture: http://2.bp.blogspot.com/_kIWY2DV0KnE/SIPw0BLYJwI/AAAAAAAAA54/NzK6hPiEAC8/s1600-h/Rear+of+Lancaster+fuselage.jpg Note the black surround to the turret has a siding door that was closed to protect the rest of the aircraft from the very cold air that entered through the open rear turret.

there was one account I read where the crew were given the order to prepare to bail out, so the rear gunner followed the procedure I outlined above. Squashed back in to the turret he kept up his observations and after a short time noticed that the fire from the engines (the cause of the prepare to bail out order) had died down, he reported this to the Pilot, no reply. Wondering if the radio system could be damaged or maybe the Pilot was just busy keeping the plane in the air he waited. After a couple more unsuccessful radio calls he finally decided to leave his post and go forwards. He was alone in the aircraft. What happened was the Pilot gave the command to bail out whilst the rear gunner had his radio unplugged, he missed the call. Astonishingly the aircraft wasn't as badly damaged as first feared and had been flying straight and level. The gunner, not knowing where he was, where the crew were or what to do next bailed out. He landed in an allied area of France and made it home, the rest of the crew had landed in enemy territory and were POWs for the rest of the war.

There are accounts of crew members being badly injured and their fellow crew rigging their parachutes to open (I'm not quite sure how this was done) and simply pushing them out in the hope that someone would find them and get them the medical attention they needed - no matter how slim it had a better chance than leaving them in a doomed aircraft.

One can hardly contemplate the experiences these young men had.

Recommended reading: Tail End Charlies - John Nichol & Tony Rennell

Bomber Boys - Kevin Wilson

Bomber Boys - Patrick Bishop

Men Of Air - The doomed youth of bomber command - Kevin Wilson

Related: Home Run : escape from Nazi Europe - John Nichol & Tony Rennell. This covers what happened after they had bailed out and how some managed to get home.

Edited for typo.

Absolutely. It was common practice at least to make provisions for the escape of crew. Of course, escaping an airplane that might be tumbling out of control might prove difficult or even impossible, crew members might be incapacitated or literally stuck in their position, making escape difficult.

There are many accounts of crews of other bombers anxiously counting the number of parachutes seen exiting from a fellow stricken bomber.

In quite a few cases where a plane was controllable but landing might be too risky, they would often return close to their home base or at least some other friendly controlled area if possible, and bail out.