
The Flying Sub is exactly that, a sub (an underwater vessel) that can also fly in the air. In other words, it is an amphibious craft.
The Flying Sub made its first appearance in the Voyage to the Bottom of the Sea second season episode "Time Bomb" and would be seen regularly in the TV series thereafter.
With the introduction of the Flying Sub, the Seaview itself needed a refit to make space for it to carry the Flying Sub. The front of the Seaview went through significant modifications with the observation nose reducing from 8 to 4 windows, and then the addition of the Flying Sub bay underneath the hull from which it could launch.
Above is a photo taken behind the scenes during filming of the Voyage to the Bottom of the Sea episode "Escape" (later called "Deadly Creature Below") with Dawson Palmer and model Astrid Shultz in front of the full size Flying Sub.
The above pictures are of an 18 inch Flying Sub model from Gary Coghlan which is very heavy and appears to be made of plaster. If you know anything about this model, please let us know.