The number 29 has some attraction since it is a prime, the sum of two squares (4*4 + 5*5), and the sum of three consecutive squares (2*2 + 3*3 + 4*4).
It is also <30, which somehow correlates with trust, at least according to Jack Weinberg, a leader of the Free Speech Movement at UC Berkeley in 1965: "Don't trust anybody over 30."
Moreover, it apparently bounds novelty: "Anything that is in the world when you're born is normal and ordinary and is just a natural part of the way the world works. Anything that's invented between when you're fifteen and thirty-five is new and exciting and revolutionary and you can probably get a career in it. Anything invented after you're thirty-five is against the natural order of things."  So, 29 seems a good compromise to keep things interesting.
There are 29 days in a leap-year February and nearly 29 days in a lunar month.
A town in California is named Twentynine Palms, Iowa was the 29th state to join the Union, and there are 29 states in India.
The Boeing B-29 Superfortress was a prop-driven heavy bomber that flew in WW II and the Korean War. It was in service with the US Air Force from 1942 through 1960.
 Douglas Adams, The Salmon of Doubt, Random House, 2002, p. 111(?).