  We present the discovery of KELT-24 b, a massive hot Jupiter orbiting a
bright (V=8.3 mag, K=7.2 mag) young F-star with a period of 5.6 days. The host
star, KELT-24 (HD 93148), has a $T_{\rm eff}$ =$6509^{+50}_{-49}$ K, a mass of
$M_{*}$ = $1.460^{+0.055}_{-0.059}$ $M_{\odot}$, radius of $R_{*}$ =
$1.506\pm0.022$ $R_{\odot}$, and an age of $0.78^{+0.61}_{-0.42}$ Gyr. Its
planetary companion (KELT-24 b) has a radius of $R_{\rm P}$ = $1.272\pm0.021$
$R_{\rm J}$, a mass of $M_{\rm P}$ = $5.18^{+0.21}_{-0.22}$ $M_{\rm J}$, and
from Doppler tomographic observations, we find that the planet's orbit is
well-aligned to its host star's projected spin axis ($\lambda$ =
$2.6^{+5.1}_{-3.6}$). The young age estimated for KELT-24 suggests that it only
recently started to evolve from the zero-age main sequence. KELT-24 is the
brightest star known to host a transiting giant planet with a period between 5
and 10 days. Although the circularization timescale is much longer than the age
of the system, we do not detect a large eccentricity or significant
misalignment that is expected from dynamical migration. The brightness of its
host star and its moderate surface gravity make KELT-24b an intriguing target
for detailed atmospheric characterization through spectroscopic emission
measurements since it would bridge the current literature results that have
primarily focused on lower mass hot Jupiters and a few brown dwarfs.
