The A-10 was designed around the GAU-8 Avenger, a heavy automatic cannon which forms the aircraft's primary armament.
In our fun-driven and picture-perfect world of combat flight simulators, aircrafts are measured with a single stick that can measure only two parameters: firepower and speed. When speed is not an aircraft's forte, we tend to ignore it as an anomaly and focus on the other feature that we love, firepower. The quote above is taken from Wikipedia and is a "fun fact" that is ubiquitous in flight sims forum discussions. That "fact" is actually a half-truth.
What is the measure of great combat aircraft?
Continuing with the measuring stick analogy mentioned above, in the real world there are as many types of sticks as types of missions doctrine can conceive. Furthermore, these many sticks may measure the same parameters in completely different scales. High speed may be great for a fighter flying an interception mission but will be a a handicap for a close air support pilot trying to drop bombs on the enemy who, according to the tactical air controller, is located in "the field north of the house with a red roof". The real measure of a combat aircraft is how well it can accomplish the mission for which it was designed.
It is rather odd that a great aircraft like the A-10 was conceived in times where the existence of its intended mission (close air support) was being debated in the USAF.
For all practical purposes, this is the best history of the conception of the A-10 available to the general public. I can't find any other book that comes close to the level of detail and quality of presentation offered by Douglas Campbell. This is not a book that I can recommend to people who are interested in combat stories or aeronautical design. This book is rather a close-up at how the A-10 came to existence despite ferocious inter-service rivalry, the US Congress and the bipolar attitude that the USAF historically had for close air support. Great weapons and their delivery means follow a rather tortuous path and Campbell's book is a rare glimpse into a world that most people haven't heard of.
As for the opening quote above, the original "concept formulation package" (CFP, dated around 1968) for the competition that the A-10 eventually won, specified a requirement for a to-be-developed 30 mm tank-killing gun. This CFP assumed very short development times and forced competitors to a schedule where both the cannon and the aircraft had to be worked simultaneously (instead of sequentially). When the time for the competition fly-offs arrived, the GAU-8 was not even ready to be mounted in the aircraft. On the meantime, the A-10 was sporting the M61A1 “Vulcan” 20mm cannon. I reckon this issue has "chicken or the egg" written all over it, but certainly nobody took a GAU-8 and built an aircraft around it.