Apple launched the App Store in July, 2008, which reached its first billion downloads within 9 months (April 23rd, 2009). Shortly thereafter, Apple announced the App Store had passed the 5 billion download mark on June 23rd, 2010.
The next milestone was 10 billion, which occurred a mere 6 months later on January 22nd, 2011. Thirteen months later it passed the 15 billion app download mark, continuing its trajectory to 25 billion downloads by March, 2012, and skyrocketing to 50 billion downloads on May 2nd, 2013.
Today, during its annual developer conference, Apple announced that the App Store has surpassed 100 billion app downloads, while also paying out $30 billion (USD) to developers.