I have a couple guys I worked with that seem to love to bash HDs. BMW lovers... and they mentioned to me that the Fat Boy was named after the A-bombs dropped on Japan during WWII. These guys told me the Harley Davidson choose this name because HD was sick of the Japan bikes trying to copy them at the time.

So I searched it on the trusty internet and it appears that indeed it is an Urban Legend but don't know the truth to it. Below is what I found from my search:

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By 1990, with the introduction of the "Fat Boy", Harley once again became the sales leader in the heavyweight (over 750 cc) market. At the time of the Fat Boy model introduction a story rapidly spread that its silver paint job and other features were inspired by the World War II American B-29 bomber; and that the Fat Boy name was a combination of the names of the atom bombs (Fat Man and Little Boy) that were dropped on Nagasaki and Hiroshima respectively. However, the Urban Legend Reference Pages lists this story as an urban legend.
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How could an international company ever admit to such a thing. And the new Fatboys have 50 cal holes in the wheels which happens to be the size of the guns on the Enola Gay. Don't worry. It's just a coincidence.

 

Let see what the other combos could have been: (Fat Man and Little Boy)

Little Man
Little Fat
Man Boy

I think the choose the right combo

 

I traveled back in time and someone spotted me oggling a new (back then) Harley that they had a hard time naming. One guy hollered, "now thats a Fat Boy!" The name stuck ever since. J/k!

 

From his brother Bob.

 

At the time of the Fat Boy model introduction a story rapidly spread that its silver paint job and other features were inspired by the World War II American B-29 bomber; and that the Fat Boy name was a combination of the names of the atom bombs fat man and little boy that were dropped on nagasaki and hiroshima respectively.

 

Willie "G" wanted to name the new bike after his wife. That got voted down because nobody wanted to ride a "Fat Bitch"

 

yep. The only motorcycle named after the very short nuclear war

 

First off IMO most BMW riders are *** hat's Second I never really put much thought into why it's called the Fat Boy. I just thought it might be called that since it has the wide front tire and forks LOL!

 

Back in the 60s we called a stripped H-D with a standard tank with the two sides and the speedo console a Fat Bob. That meant it didn't have a peanut tank. I think H-D glommed on to the enthusiasts lingo rather than the Air Corps. Where do people get their info nowadays? No imagination. Or maybe too much imagination. Fat Boy is just a short step from Fat Bob, which was the old name for a stripped Hog dresser.