▁it ▁was ▁co o led ▁with ▁a ▁Lamb lin ▁radi ator ▁mounted ▁vent r ally ▁at ▁its ▁rear . ▁ ▁The ▁emp enn age ▁of ▁the ▁D . 3 0 ▁was ▁conventional , ▁with ▁the ▁str ut - b rac ed ▁tail plane ▁mounted ▁on ▁top ▁of ▁the ▁fus el age . ▁The ▁rear ▁control ▁surfaces ▁were ▁un bal anced ; ▁the ▁rud der ▁reached ▁down ▁to ▁the ▁bottom ▁of ▁the ▁fus el age , ▁moving ▁in ▁a ▁cut out ▁between ▁the ▁elev ators . ▁The ▁single ▁main ▁whe els ▁of ▁the ▁under car riage ▁were ▁mounted ▁on ▁pairs ▁of ▁V - form ▁str uts ▁joined ▁to ▁the ▁lower ▁fus el age ▁longer ons , ▁with ▁near - vertical ▁shock ▁absor ber ▁legs ▁attached ▁to ▁the ▁wing . ▁A ▁tail sk id ▁completed ▁the ▁conventional ▁landing ▁g ear . ▁ ▁The ▁D . 3 0 ▁first ▁fle w ▁on ▁ 2 1 ▁May ▁ 1 9 3 1 . ▁A ▁second ▁prototype ▁followed ▁but ▁was ▁modified ▁into ▁a ▁trim ot or ▁aircraft , ▁designated ▁the ▁D ew oit ine ▁D . 3 1 ▁and ▁power ed ▁by ▁three ▁His p ano - Su iza ▁ 9 Q ▁nine - cy l inder ▁radial ▁engines . ▁ ▁The ▁outer ▁engines ▁were ▁each ▁mounted ▁well ▁below ▁the ▁wing ▁via ▁two ▁pairs ▁of ▁str uts . ▁A part ▁from ▁the ▁three ▁engines ▁and ▁a ▁consequ ent ▁increase ▁in ▁weight ▁and ▁slight ▁reduction ▁in ▁length , ▁the ▁D . 3 1 ▁was ▁very ▁similar ▁to ▁the ▁D