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 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 .