  The broadband spectrum from the 2013 December 20 $\gamma$-ray flare from
3C~279 is analyzed with our previously-developed one-zone blazar jet model. We
are able to reproduce two SEDs, a quiescent and flaring state, the latter of
which had an unusual SED, with hard $\gamma$-ray spectrum, high Compton
dominance, and short duration. Our model suggests that there is insufficient
energy for a comparable X-ray flare to have occurred simultaneously, which is
an important constraint given the lack of X-ray data. We show that first- and
second-order Fermi acceleration are sufficient to explain the flare, and that
magnetic reconnection is not needed. The model includes particle acceleration,
escape, and adiabatic and radiative energy losses, including the full Compton
cross-section, and emission from the synchrotron, synchrotron self-Compton, and
external Compton processes. We provide a simple analytic approximation to the
electron distribution solution to the transport equation that may be useful for
simplified modeling in the future.
