Is no one commenting on the board itself?
I'm no Eagle expert however it does not look complete to me, there is an incomplete net for the GND from the connector.
(OK I poured the polygon - see the later comment about swapping to improve it)
The mounting holes are too close to the TIP31 and IC corners, terminal block - whats going through those holes? (screws have heads, plastic pegs sometimes do too).
The TIP31's you can get away with as your renders show them standing up (could you not find a vertical version?)
Will the transistors get hot?
How does the arduino board get +power? I see no connection to it for that.
Swapping the pinout on the terminal block will make the track layout far easier, put +pwr closer to the pwr in connector to see what I mean.
This also improves the GND pour to be more continuous. (although I'm getting a big gap betweent he GND that covers the board and the GND that goes under the arduino, ought these both be solidly connected instead of a thin track?
If your not in a hurry you could use one of the fabs such as
https://www.seeedstudio.com/fusion_pcb.html who will make you 10 of these for very little (under $10)