The directional neighbor discovery problem, i.e., spatial rendezvous, is a fundamental problem in millimeter wave (mmWave) networks. The challenge is how to let the transmitter and receiver beams meet in space under deafness caused by directional transmission and reception. In this paper, we present a Hunting-based Directional Neighbor Discovery scheme, where a node continuously rotates its directional beam to scan its neighborhood for neighbors. Through a rigorous analysis, we derive the conditions for ensured neighbor discovery, as well as a bound for the worst case discovery time. We validate the analysis with extensive simulations, and demonstrate the superior performance of the proposed scheme over two benchmark schemes.