'b' in mechanical equation of Dc motor?
The b represent the drag/viscous coefficient of the motor 

http://www.pitt.edu/~qiw4/Academic/MEMS1082/DCmotors.pdf


  2.2 The mechanical equations 
  Let us now deal with the mechanical representation of the motor. It has been shown in Section 1.2 that the motor exerts a torque, while supplied by voltages on the stator and on the rotor. This torque acts on the mechanical structure, which is characterized by the rotor inertia J and the viscous friction coefficient F. It has also to be taken into account that


Also: 

Viscous damping in BLDC

DC motor differential equation