Why would aircraft engines be mounted with "toe out"?
According to this source 


  The engines were toed out slightly to improve engine-out handling;
  three tailfins were used to ensure that the aircraft could keep flying
  more or less straight if both engines on one wing went down.


I belive it was done to keep the thrust more equal in the event of an outboard engine loss, i.e. they are inline with the direction of flight when an engine loss occurs. Keep in mind that at the time these planes were designed engine failure was a [very]common thing and the mentality was a bit different than it is now. If a little bit of toe out mounting took a speed toll but made the aircraft far more controlable in the event of an engine loss it was considered worth while.  

In modern multi-engine propellor aircraft counter rotating propellors (left side and right side props spin oposite directions) are generally employed to fix this problem and avoid the critical engine issue. I do belive that some single engine aircraft still do employ a slightly canted engine position for this reason.