The B-17 is the most famous of all the World War II bombers. It was dubbed the "Flying Fortress" because of all the firepower on board. Crews loved flying the B-17 for her ability to take and withstand heavy combat damage and still get back home.
The B-17 saw action in every theatre of operation with a typical mission lasting over 8 hours.
In 1934 Boeing started construction of this heavy bomber.
Today about 12-14 Flying Fortresses can still take to the air.
(From the libertyfoundation.org's website)

Personal note: I was able to climb inside the B-17. The space is quite narrow and cramped and I had to walk along a narrow plank (over the bomb bay) and do a bit of crouching down to get to the cockpit and the tail gunner area.
The Liberty Foundation visited Tucson for 4 days with the Liberty Belle B-17 and offered rides for $425.
Tucson is a favorite visiting spot for vintage warplanes because of the sunny year-round climate.