 In March 1943, a group of airmen drawn from across the ranks of the RAF were assembled and trained for an unprecedented and potentially deadly assignment whose complete nature was not revealed to them. This largely hand-picked group, known as 617 Squadron -- including pilots from Britain, Canada, Australia, New Zealand and the USA -- were led by Wing Cmdr. Guy Gibson. They had only seven weeks to train with exercises that imitated, but never revealed, their ultimate intent: to fly deep within well-armed Germany at tree top level and destroy three dams that were essential to the Nazi steel industry. A revolutionary "bouncing" bomb that could skip across water had been developed by a visionary scientist, Barnes Wallis, but to be effective, the bomb had to be dropped from a terrifyingly close range and at very low altitude. In specially-modified Lancaster aircraft, the airmen trained for a single mission, the likes of which had never been undertaken and whose potential for success was small.