When william boeing founded the company that bears his name, what was he planning to build?
The company started in 1916 as "Pacific Aero Products Co." That statement eliminates 3 of your 4 answers. There were no jet planes in 1916. A company named "Aero Products" would not be making bicycles. There was no airmail in 1916, so no mail planes. There were contracts to build seaplanes for the military.
I think he originally built boats, then seaplanes branched off of that, then he went all airplane.