Can you put down raw hardwood flooring without tongue and grove?
Yes, if you can nail it to something (sub floor).
If on concrete or joists, a moisture barrier goes on first (first floor).
Without tongue and groove edges, hardwood flooring are abundantly face nailed.
Tongue and groove edges lock hardwood-flooring planks together along the seams, ensuring a snug fit and hiding finishing nails, but it is possible to install a wood floor without an interlocking system.