This paper presents the results of a designed experiment and confirmational testing of the bending fatigue properties of boost-diffuse gas carburized gear steels for use in heavy duty truck gearing applications. Testing was conducted on simulated gear tooth samples in unidirectional four point bending under constant amplitude load control to crack initiation. The experiment was an L32 fractional factorial with eight levels of alloy grade, four levels of shot peening, and two levels each of grinding, case depth, surface carbon, ammonia additions, and tempering temperature. The SAE 4320 and boron containing alloy grades significantly outperformed the 8600 series steels. Surface conditions highly influenced the fatigue lives with low retained austenite, low surface carbon contents, and shallow intergranular oxidation depths resulting in superior performance. The beneficial effects of grinding and shot peening the tooth root following carburizing are discussed.