In Casino Royale, they briefly go over some of his history:
’Well, in the last few years I've killed two villains. The first was in New York--a Japanese cipher expert cracking our codes on the thirty-sixth floor of the RCA building in the Rockefeller centre, where the Japs had their consulate. I took a room on the fortieth floor of the next-door skyscraper and I could look across the street into his room and see him working. Then I got a colleague from our organization in New York and a couple of Remington thirty-thirty's with telescopic sights and silencers. We smuggled them up to my room and sat for days waiting for our chance. He shot at the man a second before me. His job was only to blast a hole through the windows so that I could shoot the Jap through it. They have tough windows at the Rockefeller centre to keep the noise out. It worked very well. As I expected, his bullet got deflected by the glass and went God knows where. But I shot immediately after him, through the hole he had made. I got the Jap in the mouth as he turned to gape at the broken window.'
Bond smoked for a minute.
'It was a pretty sound job. Nice and clean too. Three hundred yards away. No personal contact. The next time in Stockholm wasn't so pretty. I had to kill a Norwegian who was doubling against us for the Germans. He'd managed to get two of our men captured--probably bumped off for all I know. For various reasons it had to be an absolutely silent job. I chose the bedroom of his flat and a knife. And, well, he just didn't die very quickly.
'For those two jobs I was awarded a Double O number in the Service. Felt pretty clever and got a reputation for being good and tough. A double O number in our Service means you've had to kill a chap in cold blood in the course of some job.