▁but ▁it ▁was ▁not ▁built ▁in ▁series , ▁because ▁Bart el ▁decided ▁to ▁design ▁an ▁improved ▁aircraft , ▁which ▁resulted ▁in ▁the ▁Bart el ▁B M ▁ 4 ▁tra iner , ▁which ▁was ▁produced ▁in ▁quantity . ▁After ▁flight ▁testing , ▁the ▁prototype ▁was ▁removed ▁from ▁service . ▁ ▁A ▁distingu ishing ▁feature ▁of ▁the ▁B M ▁ 2 ▁and ▁all ▁Bart els ▁was ▁an ▁upper ▁wing ▁of ▁a ▁shorter ▁span , ▁because ▁the ▁lower ▁and ▁upper ▁wing ▁hal ves ▁were ▁inter change able ▁( i . e . ▁the ▁lower ▁wings pan ▁included ▁the ▁width ▁of ▁the ▁fus el age ). ▁ ▁Also ▁Bart el ▁put ▁a ▁stress ▁on ▁standard izing ▁the ▁construction ▁materials ▁used : ▁steel ▁pip es , ▁metal ▁sheet ▁etc ., ▁in ▁order ▁to ▁make ▁production ▁and ▁rep airs ▁easier . ▁A ▁distingu ishing ▁feature ▁of ▁the ▁B M ▁ 2 ▁was ▁the ▁upper ▁wing ▁directly ▁over ▁the ▁lower ▁wing ▁- ▁un st agger ed ▁wings , ▁while ▁in ▁later ▁Bart el ▁designs , ▁the ▁wings ▁incorpor ate ▁forward ▁st agger ▁- ▁where ▁the ▁upper ▁wing ▁is ▁mounted ▁ahead ▁of ▁the ▁lower ▁wing . ▁ ▁Description ▁Wood en ▁construction ▁bi plane , ▁conventional ▁in ▁layout . ▁ ▁F us el age ▁rect angular ▁in ▁cross - section , ▁p lywood - cover ed ▁( engine ▁section ▁- ▁metal ▁covered ). ▁Rect angular ▁two - s par ▁wings , ▁p lywood - ▁and ▁canvas - cover ed . ▁C rew ▁of ▁two , ▁sitting ▁in ▁tan dem ▁in ▁open ▁co