Another of Silicon Valley’s biggest unicorns appears to be slowly edging toward the public stock markets — even if only by baby steps.

Airbnb announced on Thursday that it had named its first independent board member — Kenneth I. Chenault, the departing chief executive of American Express. Naming an outside board member is often an early step by a company that will eventually seek an initial public offering.

At the same time, Airbnb’s chief executive, Brian Chesky, wrote in a letter to the online room-rental service’s top hosts that he was building a company with an “infinite time horizon.”

The moves are the latest sign that another of the biggest privately held companies in the technology universe may be preparing to go public. These companies, typically valued at more than $1 billion by private investors, are known as unicorns. Dropbox, the file-storage provider, which was most recently valued at $10 billion, recently filed confidentially for an initial offering. Spotify, the music streaming giant, is pursuing an unconventional “direct listing” of its shares on the public markets.