Why was 911 chosen as the emergency phone number?

According to About.com, it was mandated that the emergency number had to be three numbers that were not already in use in the United States or Canada as either area codes or the first three digits of any phone number. The number also had to be easy to use.

The Federal Trade Commission had originally planned to have AT&T run the emergency line, but the president of Alabama Telephone, Bob Gallagher, decided to beat them to the punch and created the first working 911 emergency service, built in Haleyville, Ala. The first 911 call was placed at exactly 2:00 p.m. on Feb. 16, 1968.