You might want to read some historical articles first:
That being said, oldinux.org
has some pretty ancient stuff, including some hdd images for emulators (mostly Bochs). But that most likely won't work with VirtualBox.
As Tobi said, the earliest kernel versions weren't really distributed on CD media and at first there were no distributions. But, what most call the "first distribution" were boot/root disk images distributed by HJ Lu. You can also try out those from oldlinux.org archive
. I don't know for VirtualBox, but I think they worked on QEmu last time I tried.