  We report the discovery of GRB 221009A, the highest flux gamma-ray burst ever
observed by the Fermi Gamma-ray Burst Monitor (GBM). This GRB has continuous
prompt emission lasting more than 600 seconds which smoothly transitions to
afterglow visible in the GBM energy range (8 keV--40 MeV), and total energetics
higher than any other burst in the GBM sample. By using a variety of new and
existing analysis techniques we probe the spectral and temporal evolution of
GRB 221009A. We find no emission prior to the GBM trigger time (t0; 2022
October 9 at 13:16:59.99 UTC), indicating that this is the time of prompt
emission onset. The triggering pulse exhibits distinct spectral and temporal
properties suggestive of the thermal, photospheric emission of shock-breakout,
with significant emission up to $\sim$15 MeV. We characterize the onset of
external shock at t0+600 s and find evidence of a plateau region in the
early-afterglow phase which transitions to a slope consistent with Swift-XRT
afterglow measurements. We place the total energetics of GRB 221009A in context
with the rest of the GBM sample and find that this GRB has the highest total
isotropic-equivalent energy
($\textrm{E}_{\gamma,\textrm{iso}}=1.0\times10^{55}$ erg) and second highest
isotropic-equivalent luminosity
($\textrm{L}_{\gamma,\textrm{iso}}=9.9\times10^{53}$ erg/s) based on redshift
of z = 0.151. These extreme energetics are what allowed us to observe the
continuously emitting central engine of GBM from the beginning of the prompt
emission phase through the onset of early afterglow.
