The results of the muon-spin rotation experiments on BCS superconductors NbB_2+x (x=0.2 and 0.34) are reported. Both samples, studied in the present work, exhibit rather broad transitions to the superconducting state, suggesting a distribution of the volume fractions with different transition temperatures (T_c's). By taking these distributions into account, the dependence of the inverse squared zero-temperature magnetic penetration depth (λ₀⁻²) on Tc was reconstructed for temperatures in the range 1.5 K≲T_c≲8.0 K. λ₀⁻² was found to obey the power law dependence λ₀⁻²∝T_c^3.1(1) which appears to be common for some families of BCS superconductors as, e.g., Al doped MgB₂ and high-temperature cuprate superconductors as underdoped YBa₂Cu₃O_7−δ.