A group of survivors (likely pilots) worked on fixing up a plane (after they got a ton of supplies), they knew they were running out of time when some of them got bitten so they decided to start making drops for other survivors or else the supplies whould go to waste.
I know it sounds highly unlikely but i'm pretty sure the military whould not care about survivors.
EDIT: To go with this story, the planes could start spawning after a time interval has passed (maybe 6 months to a year)