Flying tigers are flying close by. In that picture every pilot is looking at the airplane next to him and trying to maintain his position while maintaining the same attitude as his neighbor.
The front pilot camera looking at the plane is doing the same thing. Then the planes were very slow and simple. A formation like this would be the stuff of stunt fighters today.
Flying that close was usually done in bad weather to maintain visual contact with the aircraft in front of them and for further flight.