This answer may not give you a complete answer, but it is a piece of the puzzle. The people who made the first real off road bike in America, (Enduro bikes 1st, Motocross 2nd) were the Penton family. The Father an two sons were top competitors in Enduro racing. the bikes were called Pentons, then later sold and became the KTM brand. (I know H.D. & Indian made bikes but not strictly for off-road dirt, flat-tracking excluded)
Honda
the first dirt-bike ever made was a honda. There were bike manufatures in Europe that were making dirt type bikes before Honda was even a company.
Yamaha did....
Yamaha made the first Yamaha dirt bike
DJL made the first dirt bike.
KTM
He made the first dirt bike
Dirtbikes are made everyday in Japan..........
Travis Campa in 1863.
harley Davidson
The PW50 is a dirt bike that is manufactured by Yamaha. It is branded as "The first choice for first timers." as it is made to be used by children rather than adults.
It's made out of dirt obviously hence the name dirt bike.