Updated: Dec 12, 2018
There is a pernicious myth in academia that you must have done all the work you propose to do in a grant before anybody will give you the money. This is an unfortunate symptom of failing to understand that the core task of a grant writer is not the communication of cold hard facts, but to secure a readers interest and trust.
You can build trust many ways but the best way is to do it implicitly, not explicitly. Doing explicit trust building by telling people how difficult your project is or how amazing or novel it is, has the opposite effect. It makes people say "I call BS on that".
Instead you want to build trust implicitly. How do you build trust implicitly?
By doing a great job of diagnosing the fundamental pain that people share. If you do that, and show you have an understanding of the field's trials and tribulations, you will implicitly gain their trust.
Another way to build trust is to show pilot data or prior relevant experience. This is also effective. But beware of showing too much pilot data as you risk destroying the novelty contrast in your grant. So don't just rely on pilot data to build trust. You must also always implicitly build trust by doing a great job of diagnosing the problem you are solving.
Dr Shieak Tzeng, R&D Canvas