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Fast, sensitive, and precise detection of nitric oxide (NO) is critical to many applications in environmental monitoring and early disease diagnosis via respiratory testing. An effective detection system requires a sensor to detect NO gas at the parts per billion (ppb) level, and this system should possess a high degree of anti-interference selectivity. To achieve these targets, a series of gas sensor thin films based on intrinsic WO 3 , one-additive-doped WO 3 (prepared by doping In 2 O 3 or Nb 2 O 5 ), and two-additive-doped WO 3 (synthesized by doping with In 2 O 3 and Nb 2 O 5 ) oxides were successfully grown. By analyzing the properties of sensitivity, selectivity, responsiveness, and recovery time of the gas sensors, we found that WO 3 -1wt%In 2 O 3 -1wt%Nb 2 O 5 has overwhelming advantages over intrinsic WO 3 , WO 3 -In 2 O 3 , and WO 3 -Nb 2 O 5 . A sensing response value of 2.4 was observed for NO concentrations as low as 20 ppb from the WO 3 -1wt%In 2 O 3 -1wt%Nb 2 O 5 sensor. With 100 ppb NO gas, the WO 3 -1wt%In 2 O 3 -1wt%Nb 2 O 5 sensor achieved a high response of 56.1 at 70 °C, which is a state-of-the-art performance for NO detection at low working temperature settings. WO 3 -1wt%In 2 O 3 -1wt%Nb 2 O 5 also yields significantly improved selectivity and stability over intrinsic WO 3 , WO 3 -In 2 O 3 , and WO 3 -Nb 2 O 5 . Studies on the sensing mechanism show that the grain size, rather than the n-n heterostructure effect, plays a dominant role in the observed results. By decreasing the grain size so that it is close to the thickness of the space-charge layer, the sensing response is enhanced. Although room remains to further improve the sensing properties, the performance of WO 3 -1wt%In 2 O 3 -1wt%Nb 2 O 5 is sufficient for implementation in low-content NO detection devices.