The Lancaster bombers had a small chemical toilet, so if they had time during the mission they could go use that. Part of the problem, however, was that they would be in thick clothing (multiple layers) to keep from being cold so it would have taken some time to "prepare - especially under enemy fire.

In Louis Zamperini's autobiography Devil at My Heels, he talks about how his bomber crew would stick a funnel into a small pipe in the side of the plane, and they'd piss through it.

One time, he played a prank on his mates by stuffing wads of chewed gum into the pipe's outside end before taking off.

Later, the copilot pissed and filled the funnel with urine. He danced around while gingerly trying not to spill it, when they hit some turbulence, he tripped, and it splattered all over his face. While Louis laughed watching.

They were flying B-17's in the pacific.