The fundamental vibrational frequency of the B-N stretch in BH3NH3 has eluded gas-phase experimental observation for decades. This work offers a theoretical anharmonic prediction of this mode to be 644 cm-1, using a Cartesian quartic force field at the CCSD(T)-F12/cc-pVTZ-F12 level of theory. The other fundamental frequencies reported herein have a mean absolute error of only 5 cm-1 from the seven available gas-phase experimental frequencies, making the anharmonic vibrational frequencies and rotational constants the most accurate computational data available for BH3NH3 to date. The inclusion of Fermi, Coriolis, and Darling-Dennison resonances is a major source of this accuracy, with the non-resonance-corrected frequencies having a mean absolute error of 10 cm-1. In particular, the inclusion of the 2ν6 = ν5 type 1 Fermi resonance increases the B-N stretching frequency by 14 cm-1 compared to previous work. Ammonia borane also represents one of the largest molecules ever studied by quartic force fields, making this work an important step in extending the breadth of application for these theoretical rovibrational techniques.