I believe the Lancaster bomb bay was as long,and open (not sectioned), as it was because it was designed to carry torpedoes. This was a requirement of the original Air Ministry order. Don't know whether they ever did operate with them. Not to my knowledge but someone may no better.
Steve
The Lanc was an ASW aircraft with the RCAF for many years post war and it carried torpedos if you check the log on the linl below you'll see where it practised torpedo frops Lancaster FM159 Flight Log