  [Abridged] Luminous hot stars dominate the stellar energy input to the
interstellar medium throughout cosmological time, they are laboratories to test
theories of stellar evolution and multiplicity, and they serve as luminous
tracers of star formation in the Milky Way and other galaxies. Massive stars
occupy well-defined loci in colour-colour and colour-magnitude spaces, enabling
selection based on the combination of Gaia EDR3 astrometry and photometry and
2MASS photometry, even in the presence of substantive dust extinction. In this
paper we devise an all-sky sample of such luminous OBA-type stars, designed to
be quite complete rather than very pure, to serve as targets for spectroscopic
follow-up with the SDSS-V survey. We estimate "astro-kinematic" distances by
combining parallaxes and proper motions with a model for the expected velocity
and density distribution of young stars; we show that this adds useful
constraints on the stars' distances, and hence luminosities. With these
distances we map the spatial distribution of a more stringently selected
sub-sample across the Galactic disc, and find it to be highly structured, with
distinct over- and under-densities. The most evident over-densities can be
associated with the presumed spiral arms of the Milky Way, in particular the
Sagittarius-Carina and Scutum-Centaurus arms. Yet, the spatial picture of the
Milky Way's young disc structure emerging in this study is complex, and
suggests that most young stars in our Galaxy ($t_{age}<t_{dyn}$) are not neatly
organised into distinct spiral arms. The combination of the comprehensive
spectroscopy to come from SDSS-V (yielding velocities, ages, etc..) with future
Gaia data releases will be crucial to reveal the dynamical nature of the spiral
arm themselves.
