Bulk glasses of the series (100−x)[0.4Na₂O-0.2Nb₂O₅-0.4P₂O₅]-xB₂O₃ with x = 0–48 mol% B₂O₃ were prepared by slow cooling in air. Their glass transition temperature increases within the range of 0–16 mol% B₂O₃, but further additions of B₂O₃ result in its decrease. Their structure was investigated by Raman, ¹¹B, and ³¹P MAS NMR spectroscopy. The relative number of BO₄ units decreases with increasing B₂O₃ content, while the number of BO₃ units increases up to 59 % at x = 48. The upfield shift of a broad resonance peak in the ³¹P MAS NMR spectra is ascribed to an increasing connectedness of the structural network with increasing B₂O₃ content. Strong Raman band at 916–929 cm⁻¹ shows on the presence of NbO₆ octahedra in the structural network of these glasses. With the B₂O₃ addition, a decrease in DC conductivity is observed, which is attributed to the decrease in the concentration of Na⁺ ions.