Airbnb is adding Ken Chenault, outgoing CEO of American Express, to its board, said CEO Brian Chesky in an open letter.

"Airbnb is built on trust. As the CEO of American Express, Ken has built one of the most successful trust-based companies in the world," Chesky wrote.

Airbnb is preparing for an IPO. Chenault will be the first independent member on a board that currently has its co-founders — Chesky, Nathan Blecharczyk and Joe Gebbia — sitting in three of five seats. Chenault, who has run American Express for 17 years, will step down in February.

Chenault also is the first African-American to join Airbnb's board. The board still doesn't include any women, but Airbnb told CNBC that it is in serious discussions with a number of other people about joining the board. "The next member will be a woman, and she'll be joining later this year," a spokesperson told CNBC.

Last week, Facebook announced Chenault's appointment to its board.

Chesky also said in the letter that Airbnb aims to be an "infinite company" that will "survive to see the next century, not just the next quarter."

Chesky said the company will release its first Annual Stakeholder Report in March, which will "explicitly identify the criteria by which we want to hold ourselves accountable to our stakeholders."