  To assist game developers in crafting game NPCs, we present EvolvingBehavior,
a novel tool for genetic programming to evolve behavior trees in Unreal Engine
4. In an initial evaluation, we compare evolved behavior to hand-crafted trees
designed by our researchers, and to randomly-grown trees, in a 3D survival
game. We find that EvolvingBehavior is capable of producing behavior
approaching the designer's goals in this context. Finally, we discuss
implications and future avenues of exploration for co-creative game AI design
tools, as well as challenges and difficulties in behavior tree evolution.
