Really? How long did it take to develop and deploy an atomic bomb from a cold start?
With slide rules, chalk, and all manual machine tools?
How long should it take now with high performance computers, CAD/CAM, CNC machine tools and 3-d printers?
A modern fighter is much more complex than an atomic bomb. You can make computers and Cad twice as fast but that does not make an engineer twice as productive.
What is Mil Spec on code generation? How many lines of code were written, beta tested, reviewed, corrected and released during the Manhattan project? Any idea how many lines of code are required for each variant of the F-35?
I would hazard a guess and say that a modern stealth fighter is orders of magnitude more complex than the WWII era bombs. Just the avionics software alone has millions of lines of code which has to be of much higher quality than typical software applications.