I was researching Jimmy Doolittle this morning, and I found this on History.com:
To aid his record-breaking 1922 coast-to-coast flight, U.S. military strategist Jimmy Doolittle invented a funnel-and-tube-based "pilot dehydrator"—possibly the earliest airplane toilet.
That led me to the question: What did they do before 1922 when they felt like they had to go?