Thanks for the thoughtful response. Obviously I won't be able to do justice to an entire business plan in a few sentences.

They are definitely not representing their order book as the basis for a full debt financing. I appreciate your assuming best intent as I use broad brush strokes in the parent comment and below.

A few thoughts:
+ I think the history of startups is also one of teams succeeding with far less capital than the incumbents. They're on track to build the fastest civilian aircraft ever built for less than $100M raised which speaks to their accomplishments to date. Aero is notorious for cost overruns but their capital efficiency to date is impressive.

+ Of course, the biggest risk is that there are cost over runs due to unforseen issues, e.g. Bombadier's C-Series estimated to cost $2B and that ended up closer to $5B (though that was also for multiple planes and configurations). At the same time the A320-NEO was done for below $1.5B I believe. Time will tell how much it actually costs but I think it's probably much closer to $2B than say $20B.

+ You are correct that these are not pre-orders. I shouldn't have used that term and should have used LOIs (and now I can't figure out how to edit the comment). As I understand it, the LOIs are of varying levels of commitment. The earliest LOIs were non-binding and with no skin in the game. More recent LOIs have more teeth. The strategy has been very clever. Each batch of LOIs has terms more favorable to Boom, so there is an incentive to move before the LOIs become less favorable to you (the airline). And there is a competitive dynamic that is at work that engages companies, e.g. JAL vs ANA, via incentives such as exclusivity on certain routes or deal sweeteners like the opportunity to invest. I think it's unlikely that 100% of these LOIs convert as external circumstances will always be a factor (e.g. airline gets a new CEO who has a different strategy) but the newer LOIs also have significant executive, CEO, and board buyin from the airlines, so they're certainly not throwaway. As you noted, airlines are very conservative about basically everything. The reason these LOIs are managing to get board level approval is that the math makes a ton of sense if the planes fly. You're essentially replacing less profitable narrow body jets with a Boom jet on certain routes so you stand to make more money as an airline if you get one of these.

+ It's much harder to debt finance the entire thing as an R&D endeavor at once but there are multiple ways to traunch this and phase it in over time. And not all of the costs need to borne by Boom directly. It's pretty standard for suppliers (not the airlines, as you noted) to put up significant commitments as part of the development process and bear those costs and risks. Many of the existing partners for major components of the Boom plane are bearing these costs directly already, because the math behind unlocking supersonic makes sense and even if the plane doesn't launch they stand to benefit from the R&D on these new components. Sharing these costs such that the partners benefit from the upside in a success case (huge new market the supplier is a leaders in) and in a failure case (IP that makes their existing components much better), while also bearing the capital risk helps reduce the capital and debt burden on Boom itself.

Anyway, your read that this is a well-intentioned aerospace project full of great engineers trying to do something wildly ambitious is correct. Anything I've misstated that might imply otherwise is a failure on my part, not Boom's. I also think they are as savvy on their business as they are talented at the engineering, which is why they've even made it this far.