Which is exactly what I said. 
No, this is what you said.
American aircraft were numbered sequentially by date of acceptance for service (ie. P-36, P-38, P-39, P-40, P-47 and P-51 are oldest to youngest sequentially), and the letter designates its primary use- P for pursuit (fighter), B for bomber, C for cargo, X for experimental, etc.
The USAAF aircraft were not numbered by date of acceptance but by model number ordered. The only think you were correct on was the letter designation.
ack-ack
