31Tr is derived from SOR NB 18 city bus.

It is made of two rigid sections linked by a pivoting joint.

Body of the vehicle is welded from steel-voltage profiles, flashings from the outside and interior are lined with plastic sheeting.

The floor of the bus is at a height of 360 mm above the ground.

In Czech and Slovak cities they replaced old high-floor trolleybuses Škoda 15Tr