A search for the supersymmetric partners of the Standard Model bottom and top quarks is presented. The search uses 36.1 fb$^{-1}$ of pp collision data at $ sqrt{s}=13 $ TeV collected by the ATLAS experiment at the Large Hadron Collider. Direct production of pairs of bottom and top squarks ( $ {overline{b}}_1 $ and $ {overline{t}}_1 $ ) is searched for in final states with b-tagged jets and missing transverse momentum. Distinctive selections are defined with either no charged leptons (electrons or muons) in the final state, or one charged lepton. The zero-lepton selection targets models in which the $ {\overline{b}}_1 $ is the lightest squark and decays via $ {\overline{b}}_1\to b{\overline{\chi}}_1^0 $ , where $ {\overline{\chi}}_1^0 $ is the lightest neutralino. The one-lepton final state targets models where bottom or top squarks are produced and can decay into multiple channels, $ {\overline{b}}_1\to b{\overline{\chi}}_1^0 $ and $ {\overline{b}}_1\to t{\overline{\chi}}_1^{\pm } $ , or $ {\overline{t}}_1\to t{\overline{\chi}}_1^0 $ and $ {\overline{t}}_1\to b{\overline{\chi}}_1^{\pm } $ , where $ {\overline{\chi}}_1^{\pm } $ is the lightest chargino and the mass difference $ {m}_{{\overline{\chi}}_1^{\pm }}-{m}_{{\overline{\chi}}_1^0} $ is set to 1 GeV. No excess above the expected Standard Model background is observed. Exclusion limits at 95% confidence level on the mass of third-generation squarks are derived in various supersymmetry-inspired simplified