Facts:
Flamethrowers are not firearms.
Flamethrowers are not destructive devices.
Flamethrowers are not explosive devices.
On the federal level, they're completely legal.
But some states may have laws about them, I don't know.
A friend of mine owns one that is perfect and like new and has never been "fired", pardon the pun. All he needs is a set of correct tanks and he'll pull a burn permit and light the sucker off at a small firebug party, with the fire department standing by. He also has the aluminized fire clothing worn by airport firemen, which he will be wearing "just in case".
This same friend also owns a Marine Corps surplus HMMWV (It's genuine) and an early Colt M16, so early it's marked AR-15. It's former Air Force property and is one of the few transferrables of its type.
CJ