StephenH

Let me offer up some additional comments:
1) In additional to dealing with weight variations, you have height variations. It may not be obvious, but if you're, say, 5'-10", you can go down to Walmart and buy a bike and it'll fit you reasonably well. And if you're 6'-2", it just won't. So to complicate the supply of frames, you have deal with larger/smaller frames as well.
2) If you are actually designing the bike, you'd want to establish an upper bound as well. That may be limited by something mundane like the available seatposts, cranks, pedals, or brakes. Or may be limited by finding out that nobody weighing over, say, 600 lbs, has ever ridden a 2-wheel bike (I'm just guessing at what that limit would be, I haven't taken a poll.) However, that maximum weight will determine a lot of the design parameters for the product.
3) Bikes aren't normally built for 400-lb people, but they are quite commonly built for 400-lb teams, so research tandem wheels as a source of additional components. (Main difference: I think tandem rear wheels are wider, so you build the frame wider to handle them.)
4) A question to consider is how many people weighing over 350 lbs have put a lot of miles on a bike at that weight? I'm sure it's been done. But I would guess most riders of that weight range either don't ride very much, or they give up riding, or they lose weight and fall out of that weight range. So something along the lines of a bike rental program may make more sense that actually designing a bike. And designing a $2,000 bike may be very limited in returns due to those kinds of issues.
5) You can make a bike from steel, aluminum, titanium, carbon fiber, whatever, and make it as strong or as weak as you'd like. One consideration you may want to look at, though, is your available labor and materials. When I was in college, our ME lab had guys there that could weld. Could they weld aluminum, titanium, anything you put in front of them using any process whatever? I kind of doubt that, they were skilled, but not skilled in every conceivable way. Then also, how available is oddball gauge titanium tube? Or do you have the means to heat-treat a frame after welding? Working out what is and isn't feasable for materials, labor, and processes may vastly simplify the choices of what to use. For a production bike, you'd need to visit with the people in Taiwan that build production bikes.