John Loud invented the first ball point pen in 1888. He patented a pen with reservoir of ink and a ball inside the tip. He used it to mark thick line on his leather hides. It could not be used on paper.
Several improvements were made. But there were plenty of flaws which prevented its commercial exploitation. The diameter of ball should be exact, so that it should not enter the tube and block the flow. Nor it should small so that it goes out of the nib.
László and George Biro brother, one newspaper editor and other chemist, in 1935 improved the version. They used a tube as reservoir (as same as before) of ink and created a socket. The ball bearing was placed in the socket so that it rotated freely. It carried the ink from the refill and printed it on the paper. This ensured a free and uniform flow of the ink from the tube as the tip moved. These pens were named as Biro pens.
During wars, British Royal Air Force needed a new type of pen. One which could work even in high altitudes. And Biro pens gave a perfect solution to this problem. They did not leak and due to ball and socket mechanism, the ink could be pulled out of cartridge even when gravity is low due to its rotation of the ball. Their performance for Air force brought Biro pens to limelight.