Various ways to look at this.
When I used to teach aircraft design, I taught that you start from four pillars of design...
(1) What is the best available technology solving similar problems?
(2) What regulations do you have to meet?
(3) What is the operating environment?
(4) What are the role requirements for the aeroplane?
Once you got a good understanding of that (which is, actually, a LOT of work), you would then identify what - if any - new R&D was needed.
From there, the design process is reasonably inevitable and good engineers will get to the right design solution with nothing more than hard work and enough man-years.
G