After instructing Wallis to prepare his bouncing bombs, Sir Charles Portal asked Air Chief Marshal Sir Arthur Harris, Commander in Chief of Bomber Command to form a 'Special' Squadron from amongst his existing Air Groups to conduct the raid. Sir Arthur selected the 24 year old Wing Commander Guy Gibson DSO, DFC and Bar to command the new specially formed Squadron. 617 Squadron, as it was named, trained intensively for several weeks in low level, night flying and navigation. To successfully destroy the dams the Lancasters had to release the bouncing bomb at right-angles to the dam wall, from an altitude of 60 feet, at a speed of 220 mph, between 425 and 475 yards. Gibson was not told for some weeks of the exact details of Operation Chastise, as the mission was called. His squadron were not told the details until the night before the raids.
For maximum effectiveness, the raids had to take place on a clear moonlit night to aid navigation and be as close to 19 May as possible as this was when the reservoirs were at their fullest. Tests for Barnes Wallis bomb took place at Reculver Bay in north Kent and were only completed one day before the raids actually took place on May 16.