Development of what? Airframe? Electronics? Mechanical systems?

I know a couple fairly senior designers and they start with a full-size aircraft and work it down to scale. If the model is unstable they start toying with airfoil and horizontal & vert. stab. dimensions. Once the bugs are worked out the plans are offered through one of the magazine plans services. Modern types use CAD programs and design components ready to be laser-cut into prototypes almost completely off the PC.

If you look currently on the "Sport Flying" section here you can see where someone is overthinking a simple stik variety aircraft ad nauseum. Marketing is by beating a dead horse on a free forum.