[AndyJackson]

Funny thing about the A-10. It’s principal designer was the infamous Col. John Boyd. Now Boyd his famous for his theories of maneuverability leading to the F16 and derivatively the F15 and F18 (though the latter two were heavier and less maneuverable than he believed you needed). So he was branded as an advanced fighter advocate in an Air Force that wanted to do strategic bombing. Then they asked for a good ground attack, direct support aircraft. He looked at what attributes were needed (firepower, survivability) and voila the A-10 a great airplane for its mission.

[yarddog]

I have read that they asked for and received input from Hans Rudel.

[gaijin]

Hans Ulrich-Rudel was also consulted during the A-10 project:

From Wiki:

"During the war, Rudel was credited with the destruction of 519 tanks, as well as a number of ships. He claimed 9 aerial victories and the destruction of more than 800 vehicles of all types. He flew 2,530 ground-attack missions exclusively on the Eastern Front, usually flying the Junkers Ju 87 "Stuka" dive bomber, and 430 missions flying fighter aircraft. Rudel was the most decorated German serviceman of World War II receiving the Knight's Cross of the Iron Cross with Golden Oak Leaves, Swords, and Diamonds in January 1945; the decoration was created specifically for him."

Some say he was the greatest combat pilot of all time.

He was a puzzle, since when he first began he earned very low ratings and was almost drummed out of service.