While tungsten is well known for having a single equilibrium crystal structure, the body-centered cubic (BCC) α phase, a metastable β phase with the A15 structure has been extensively observed in tungsten thin films. Tungsten thin films containing the β phase exhibit unique properties, such as high-temperature superconductivity and the giant spin Hall effect (GSHE), making them promising for applications in advanced electronics. However, the study and utilization of this phase has been hindered by: (1) Unclear and inconsistent deposition protocols for reliably fabricating 100% β. (2) Limited understanding in its thermal stability, including the specific conditions and atomistic mechanisms for its irreversible transformation to the stable α phase upon heating. (3) Huge stress variations with temperature that can lead to failure during device processing. (4) A general lack of knowledge on many of its fundamental material properties. To address these challenges, we have: (1) Demonstrated that high-quality β-W films with known phase and elemental compositions can be reliably and reproducibly fabricated by introducing nitrogen during sputter deposition, and established the quantitative relationship between nitrogen content and β phase volume fraction in as-deposited films. (2) Shown that increasing nitrogen content enhances the thermal stability of β-W, developed an analytical model describing the β–α phase transition in the presence of nitrogen, extracted relevant activation energies, and explained the atomistic mechanism responsible for nitrogen stabilization. (3) Systematically characterized film stress in β-W as a function of temperature and nitrogen content using high-resolution in-situ substrate curvature measurements, and linked the observed thermomechanical behavior to underlying microstructural and compositional changes. (4) Used thermoelastic stress-temperature data from substrate curvature measurements to accurately determine the coefficient of thermal expansion (CTE) and biaxial modulus of β-W as functions of nitrogen content, with errors below 3%, and further verified these results through molecular dynamics (MD) and ab-initio molecular dynamics (AIMD) simulations.