MnxB100-x alloys (x = 30-60) were fabricated by arc melting technique. Structural analysis showed that alloy with Mn55B45 atomic composition has a single-phase orthorhombic FeB-type MnB structure. As-synthesized alloy has a saturation magnetization of 130 emu/g at 300 K, which is the largest value previously reported. Saturation magnetization increases rapidly from 3.5 to 130 emu/g as x increases from 30 to 55. Further increase of the x causes the reduction of the saturation magnetization to 50 emu/g. Magnetic properties of the MnB alloy have been also evaluated by first-principles density functional theory calculations. Electronic structure calculations were performed using full potential linearized augmented plane wave method and a spin polarized generalized gradient approximation. A 1.914 mu(B) magnetization per MnB unit was calculated, which corresponds to 158 emu/g at 0 K. Curie temperature (T-C) of the Mn55B45 was measured as 566 K which is in good agreement with the values reported for MnB alloy.