Tested on the P-38 back in the 1940-41 era
Not strictly true as the P-38 props rotated outwards giving it two critical engines and making engine failures on take off a very brief and messy affair. They rotated outwards to provide stability for the nose cannons - engine criticality wasn't a consideration, obviously!
The A400M props rotate towards each other, as BEags says, to reduce interference vibration and noise and to reduce the yawing thrust effects of assymetric flight.