  Upcoming observations of the 21-cm signal from the Epoch of Reionization will
soon provide the first direct detection of this era. This signal is influenced
by many astrophysical effects, including long range X-ray heating of the
intergalactic gas. During the preceding Cosmic Dawn era the impact of this
heating on the 21-cm signal is particularly prominent, especially before spin
temperature saturation. We present the largest-volume (349\,Mpc
comoving=244~$h^{-1}$Mpc) full numerical radiative transfer simulations to date
of this epoch that include the effects of helium and multi-frequency heating,
both with and without X-ray sources. We show that X-ray sources contribute
significantly to early heating of the neutral intergalactic medium and, hence,
to the corresponding 21-cm signal. The inclusion of hard, energetic radiation
yields an earlier, extended transition from absorption to emission compared to
the stellar-only case. The presence of X-ray sources decreases the absolute
value of the mean 21-cm differential brightness temperature. These hard sources
also significantly increase the 21-cm fluctuations compared the common
assumption of temperature saturation. The 21-cm differential brightness
temperature power spectrum is initially boosted on large scales, before
decreasing on all scales. Compared to the case of the cold, unheated
intergalactic medium, the signal has lower rms fluctuations and increased
non-Gaussianity, as measured by the skewness and kurtosis of the 21-cm
probability distribution functions. Images of the 21-cm signal with resolution
around 11~arcmin still show fluctuations well above the expected noise for deep
integrations with the SKA1-Low, indicating that direct imaging of the X-ray
heating epoch could be feasible.
