Low-hanging fruit.
I'm gonna say that this bike or a derivative of it was 'the first.'
Tooley's Patent Bi-Car (1903)
The seat is behind the gas tank , front-to-back, but the gas tank is definitely
below the level of the seat.
Followed by a licensed version the Zenith Bi-Car (1905) Hub-center steering, like the Yamaha GTS 1000!
The Militaire is a later derivation, as-was the Ner-A-Car (below).
This definitely has the gas tank under the seat. Circa 1918. That beats the Honda Gold Wing by 57 years. Another pic:
In 1922, 'Cannonball' Baker rode one 3,364 miles from the East Coast to the West Coast: Staten Island NY to L.A. CA in 7 days. Yes, he's
"that" Cannonball Baker, after-whom the apocryphal
Cannonball Run was named, the race started by
Car and Driver magazine and later turned into a successful movie franchise. Who can't forget the boob-off between voluptuous Adrienne Barbeau and equally-lovely Valerie Perrine, the former in her Lamborghini Countach, and the latter in her law enforcement officer uniform?
Dan Gurney's famous line, after winning the race in a Ferrari Daytona:
"At no-time did we exceed 170 mph!"