The secret drawings and letters belonged to the co-pilot of Enola Gay bomber which dropped the bomb and reveal Captain Robert Lewis's immense guilt following the mission. He wrote a letter following the bombing in which he recounts: 'My God, what have we done? How many did we kill? 'I am certain the entire crew felt this experience was more than anyone human had ever thought possible. 'If I live a hundred years I'll never quite get those few minutes out of my mind.' Scroll down for video . Historic: A previously unseen hand-drawn flightplan of the mission to drop an atom bomb on Hiroshima is now for sale . Bomber: The remarkable documents which include the flightplan, an eyewitness account of the bombing and log books were compiled by the co-pilot of the Enola Gay bomber which carried out the mission . Destruction: An estimated 150,000 people died when the US bombed Hiroshima, obliterating as much as 70 per cent of the city (pictured in 1945) Pilot: The incredible lot is being put to auction by the son of Captain Robert Lewis (pictured) who was on the Enola Gay that fateful day . The collection also includes a hand-drawn diagram, the pilot of the plane that bombed Hiroshima was told to arrive at 30,000ft, drop the payload two miles short of the city, then veer away sharply at 28,500ft. The previously unseen documents used to plan the dropping of the world's first atomic bomb on Japan on August 6, 1945, in a bid to end the Second World War have emerged for sale 70 years on. They are among a £300,000 archive compiled by Captain Robert Lewis, co-pilot of the Enola Gay B29 bomber, which also includes his flight logs and report of the bombing raid. One plan, drawn in pencil and coloured inks on a single sheet of blue graph paper, shows how the Enola Gay would approach Hiroshima, drop the atom bomb – nicknamed Little Boy – and then turn 150 degrees to the right to return. The plan also shows the air burst at under half a mile up over the city and the subsequent shockwaves that the explosion would create. A second plan, titled 'survival manoeuvres', expands on the initial diagram to show the flight paths of support planes The Great Artiste and Necessary Evil. The diagram concludes with a handwritten note: 'Caution – stay at least 2/3 miles away from the atomic cloud at all times.' Auctioneer Bonhams expects the drawings to fetch £26,000 in two lots when they go under the hammer as part of Captain Lewis's archive of material in New York on April 29. The collection, being sold by his son Stephen, also includes a copy of Lewis's unofficial eye-witness account of the Hiroshima mission, which he disguised as a letter to his parents, which alone could sell for £40,000. Also for sale are Lewis's two log books from the war noting flightplans, duration, type of plane, engines, and horsepower with a column for remarks. They are tipped to fetch £130,000. Rare: Lewis's unofficial eye-witness account of the Hiroshima mission, which he disguised as a letter to his parents, which alone could sell for £40,000 . War: Three days after the bombing of Hiroshima (pictured in 1945), the US inflicted similar devastation on the industrial city of Nagasaki . Eyewitness: Also for sale are Lewis's two log books from the war noting flightplans, duration, type of plane, engines, and horsepower with a column for remarks . Expensive: Manuscript flight logs belonging to Lewis, co-pilot of Enola Gay. are expected to sell for £100,000 to £130,000 . An estimated 150,000 people died when the US bombed Hiroshima, obliterating as much as 70 per cent of the city. Three days later the US inflicted similar devastation on the industrial city of Nagasaki. The two missions brought about the Japanese surrender. Unlike Enola Gay commander Captain Paul Tibbets and navigator Theodore Van Kirk, Captain Lewis, who died in 1983, chose not to cash in on his fame in the post-war years. Only one item belonging to him has been sold before – his log from the return from Hiroshima in which he wrote: 'My God, what have we done?'
The hand-drawn flightplan for bombing mission on Hiroshima is now for sale .
Unseen documents were kept by co-pilot of plane that dropped atom bomb .
Captain Robert Lewis was in the Enola Gay B29 bomber on August 6, 1945 .
Remarkable drawings put to auction by his son expected to fetch £300,000 .