Quoting from memory:
- Nine women cannot have a baby in one month.
- Adding more developers to a late project makes it later.
The problem is that in order for throwing a thousand engineers at the problem to be time-effective, each one must communicate what he has learned to all of the others. That takes time and adds friction.
Making many different aircraft prototypes simultaneously is certainly parallelizable, although it obviously would be enormously expensive.
Building prototypes in parallel does not provide the benefit of a feedback loop, as the iterative process does.
Let's say you have a 10% probability of success on the first attempt, which improves by 40% after each experiment (to 14%, 19.6%...), and you have resources for 5 attempts. The chance of getting at least one successful solution is: parallel ~ 40.95%; iterative ~ 72.19%