Flyout has taken off into Early Access on Steam, offering a Windows game that allows you to design and fly your own aircraft (thanks JDreyer). This includes free-form fuselage and wing design tools to allow you to channel your inner Howard Hughes, presumably without all the pee bottles. The Cinematic Announcement Trailer shows off the editors and some creations in flight. Here's word on the game along with a warning that at this point it's not going to hold your hand:

ABOUT THIS GAME
Flyout is an aircraft building game with an emphasis on design freedom and realistic physics, sound and effects. Use the in-game modeling tools to create an aircraft of any shape, add and configure engines for your needs, set up control surfaces and take your design on a test flight around an earth-sized procedurally generated planet.

This game is not easy. There are no in-game tutorials and tool tips are limited due to Early Access status.
Fuselage editing is similar to modeling in any 3D software, although much simplified.
There will be bugs due to the highly unrestricted editor and Early Access status of the game.
Basic understanding of aircraft physics and flight controls is required or at least recommended.