F ▁F lying ▁Fort ress , ▁and ▁later ▁a ▁single ▁Cons olid ated ▁B - 2 4 D ▁Liber ator ▁to ▁each ▁have ▁ 1 4 ▁to ▁ 1 6 ▁Brow ning ▁AN / M 2 ▁. 5 0 ▁cal ▁machine ▁guns ▁as ▁the ▁Bo eing ▁Y B - 4 0 ▁F lying ▁Fort ress ▁and ▁Cons olid ated ▁X B - 4 1 ▁Liber ator ▁respectively ; ▁each ▁meant ▁to ▁accomp any ▁regular ▁heavy ▁bom ber ▁form ations ▁over ▁occupied ▁Europe ▁on ▁strateg ic ▁bomb ing ▁ra ids ▁for ▁long - range ▁esc ort ▁duties ▁as ▁" f lying ▁destroy er ▁gun ship " s ▁The ▁Y B - 4 0 ▁was ▁sometimes ▁described ▁as ▁a ▁gun ship , ▁and ▁a ▁small ▁ 2 5 - air craft ▁batch ▁of ▁the ▁B - 1 7 - der ived ▁gun ships ▁were ▁built , ▁with ▁a ▁dozen ▁of ▁these ▁deployed ▁to ▁Europe ; ▁the ▁X B - 4 1 ▁had ▁problems ▁with ▁stability ▁and ▁did ▁not ▁progress . ▁ ▁Att ack ▁aircraft ▁ ▁During ▁World ▁War ▁II , ▁the ▁ur gent ▁need ▁for ▁hard - h itting ▁attack ▁aircraft ▁led ▁to ▁the ▁development ▁of ▁the ▁heavily ▁armed ▁gun ship ▁versions ▁of ▁the ▁North ▁American ▁B - 2 5 ▁Mitchell . ▁Four ▁hundred ▁five ▁B - 2 5 G ▁were ▁armed ▁with ▁a ▁ 7 5 ▁mm ▁( 2 . 9 5 ▁in ) ▁M 4 ▁can non ▁and ▁a ▁thousand ▁B - 2 5 H ▁followed . ▁The ▁H ▁models ▁moved ▁the ▁d