This drink is believed to be created in New Orleans in the 1960's but, it actually is a very recent drink. Although, the drink may derive from new orleans. Here is an early printing of this drink:
New Orleans for dummies by Kevin Forest Moreau in 2003
"If you stay, try the flaming Dr. Pepper shot. Only open Friday and Saturday nights; $2 cover charge."
The drink appears in literature several times after this, even today as seen here:
A Journey To Cocktail Enlightenment - Page 170 by John W Raibikis, Mike Ferron in 2010
"(12-19-08) Flaming Dr. Pepper 79"
It is clear where the drinks name derives, the final result flavor is that of Dr. Pepper. This drinks ingredients are rarely argued and has called for the same composition and preparation since it's creation.
This drink dates between 1999 - 2001 and contained at least as of then:
Layer these ingredients in a shot glass.
Amaretto (Di Saronno)
Ignite the 151 and drop into a pint glass halfway filled with beer.