Leviathan by Scott Westerfeld

What if things had been quite different at the edge of World War I and the assassinated Archduke had had a son who escaped in a walking tank?...The how about throwing in a talented girl disguised as a boy on the crew of a flying whale-ship, Leviathan?  The two plots come together as the gigantic Leviathan crash lands near the hiding place of the fleeing prince.  Fans are awaiting the sequel.