Wait a minute, because one of you got burnt and lost 6 months of work, now you won't do any work? With today's technology, I have to think there's a good middle ground between 6 months of dev work and paper prototype only.
If I was one of your prospects, I would never sign a letter of intent based on drawings only. I'd make you come back later with something, anything I could play with for 2 reasons:
1. I'd want to see that you can actually produce something, no matter how limited.
2. There's a huge difference between playing with something and talking about something. We'd arrive at a real functional requirement much faster with a working model. Anything less is just waterfall analysis and design, and we already know how well that works.
Come back when you have something real to show. Until then you're no different from any other poser.