at the moment I think it is if you kill someone on the ground.
Not exactly.
No kills whatsoever play a role in your bomber score/rank. You only have the scoring categorie of damage/sortie, damage/death, hit% vs objects, damage score points (again vs objects only) and base captures.
On the score sheet in game for some odd reasons the kills of vehicles do show, while on the website score only the kills on planes show up (at the bottom) - but both are just statistical information without any impact on your actual scoring.
