, ▁primarily ▁as ▁Oper ational ▁Support ▁A ircraft ▁( OS A ) ▁for ▁Air ▁National ▁Guard ▁flying ▁wings ▁and ▁as ▁naval ▁air ▁station ▁" station ▁aircraft " ▁until ▁ 1 9 9 0 . ▁The ▁C - 1 3 1 ▁was ▁primarily ▁replaced ▁by ▁the ▁C - 9 ▁Night ing ale ▁in ▁regular ▁US AF ▁service , ▁with ▁the ▁Air ▁National ▁Guard ▁replacing ▁their ▁OS A ▁with ▁C - 1 3 0 ▁Her cules ▁aircraft ▁and ▁the ▁Navy ▁with ▁C - 1 2 ▁Hur ons . ▁ ▁In ▁ 1 9 5 9 , ▁a ▁C - 1 3 1 ▁was ▁the ▁first ▁aircraft ▁to ▁be ▁used ▁as ▁a ▁reduced - gravity ▁aircraft ▁or ▁' v om it ▁com et ', ▁for ▁astr onaut ▁training ▁as ▁part ▁of ▁Project ▁Mercur y . ▁ ▁A ▁Sam ar itan ▁was ▁the ▁first ▁aircraft ▁used ▁as ▁a ▁flying ▁gun ship ▁test bed ▁in ▁mid - 1 9 6 3 , ▁in ▁a ▁program ▁known ▁as ▁" Project ▁T ail chas er ". ▁A ▁C - 1 3 1 B ▁( AF ▁Ser . ▁No . ▁ 5 3 - 7 8 2 0 ) ▁was ▁given ▁a ▁guns ight ▁for ▁the ▁side ▁window , ▁but ▁instead ▁of ▁guns ▁it ▁had ▁cam eras ▁in ▁the ▁cargo ▁area . ▁Event ually ▁the ▁C - 1 3 1 ▁was ▁fer ried ▁to ▁Eg lin ▁A FB ▁in ▁Florida ▁and ▁a ▁General ▁Electric ▁SU U - 1 1 A / A ▁ 7 . 6 2   mm ▁G at