From Boom's Wikipedia page, they seem to have built zero airplanes which even attempted to take off. That includes their 1/3 scale "technology demonstrator" test plane - which was supposed to fly back in 2017.

I'll guess that American's "non-refundable deposit" for the first 20 Boom aircraft was pretty small, and came out of American's marketing budget. Or was a negotiating tactic, to help American get a better price from some real aircraft manufacturer.