This only works in the recently released version 2018 right now, since Mixed Mode Shading is necessary.

1] You may have to start a new window (Window menu> New Window) since the View toolbox has to be open, which causes views to switch back to their default. The new window will open in perspective.
2] Turn on Mixed Mode Shading (ie, shaded with outlines).
3] Change the 'horizontal angle' and 'vertical angle' to get the view you want (0/0 is front, 0/90 is right, 270/0 is left, 180/0 is rear, 0/270 is bottom or use Y to orbit, for non-flat views).
3] In the View Toolbox set the View Distance to 1 US billion (1 with nine zeros).


The shaded view will start to display interior lines. At that point the depth of the visibility will depend on the size of the model. Increasing the view distance will show more depth but may also cause some items to not appear in the correct place in non-flat views.

Bonus: some items will display in color if selected and may remain after deselected (ie, not only x-ray but also shaded highlight of chosen items)