It’s called the Cormorant and the initial development has been given to Lockheed Martin’s Skunkworks.
Because nuclear subs don’t have much to do in today’s environment they want to modify the launchers to eject these aircraft. This obviously comes with problems…

The Skunk Works’s answer is a four-ton airplane with gull wings that hinge around its body to fit inside the missile tube.

The craft is made of titanium to resist corrosion, and any empty spaces are filled with plastic foam to resist crushing. The rest of the body is pressurized with inert gas. Inflatable seals keep the weapon-bay doors, engine inlet and exhaust covers watertight.