GO:0050051:   leukotriene-B4 20-monooxygenase activity
Catalysis of the reaction: (6Z,8E,10E,14Z)-(5S,12R)-5,12-dihydroxyicosa-6,8,10,14-tetraenoate + NADPH + H+ + O2 = (6Z,8E,10E,14Z)-(5S,12R)-5,12,20-trihydroxyicosa-6,8,10,14-tetraenoate + NADP+ + H2O.