From Warhammer 40k - Lexicanum
Flyin' Fortresses are a type of Ork Bommer, that is formed when two separate aircrafts are welded together.
This occurs when the Orks build an aircraft with a massive engine, to make it go really fast, but then realize too late that there is no space for its pilot and all of its guns. So in true Ork fashion, they simply wield another aircraft to its side. This results in Flyin' Fortresses having an asymmetrical design, dual fuselages and having their six Ork crew members crammed into the aircraft section without the massive engine.
The Flyin' Fortress was created by the Games Workshop employee Colin Grayson and was featured in White Dwarf 396 (UK).
- 1: White Dwarf 396 (UK), pgs. 86-87 - Parade Ground