This phase Ib/II trial is aimed at studying the combination of a drug named Selinexor (selective inhibitor of nuclear export) in combination with standard therapy for B cell Non-Hodgkin's lymphoma called R-CHOP. The investigators will establish maximum tolerated dose of Selinexor in combination with RCHOP and also study the efficacy of this combination for therapy of B cell Non-Hodgkin's lymphoma. Giving Selinexor plus chemotherapy may work better in treating patients with B cell non-Hodgkin lymphoma.