  Recent experiments have demonstrated Rabi-oscillations, superradiant pulses
and stimulated emission from negatively-charged nitrogen-vacancy
($\mathrm{NV}^{-}$) center spins in microwave resonators. These phenomena
witness the kind of collective and strong coupling which has been prerequisite
for observation of superradiant lasing in the optical frequency regime. In this
article, we investigate the possibility to employ coherence, present in both
the collective $\mathrm{NV}^{-}$ spin ensemble and the microwave field, to
achieve a superradiant maser. Our calculations show that a superradiant maser
with a linewidth below millihertz can be achieved with moderate kilohertz
incoherent pumping of over $10^{14}$ spins kept at low temperature. We show
that the superradiant masing prevails in the presence of inhomogeneous
broadening, and we present numerical and analytical studies of the dependence
of the phenomenon on the various physical parameters.
