It will not fit into the overhead luggage compartment on an airplane, but Upton Bass has developed a travelling acoustic string bass that nests into a rather (considering the contents) small utility case.  Assembles in about 15 minutes.  Think of the body of a string bass as sections of a Russian nesting doll, and you get the idea.  I don't know if the bow collapses so that it will fit into the case as well (if not, I know how they could do it).