In other words, we know that Bond entered the rough and violent life of the military at age 17, but we don't know how he made his first forays into the realm of espionage and how he came to become a spy ripe for recruitment to MI6.
Actually Bond isn't really depicted as a spy. At least not as a professional one. He drifted into that twilight zone between intelligence and sabotage work that was the realm of the commandos and of SOE at the beginning of the war (taking here Fleming's own 'revised' timeline for Bond), probably because he was athletic, daring and was fluent in the language of the enemy, German, and of occupied France.
After the war the likes of Bond were normally quickly demobilised, their outfits either disbanded or integrated into the 'regular' Secret Service (that came under the roof of the Foreign Office, not the Ministry of Defence, as Fleming's version was described as). Bond apparently is a remnant of that time and some of it actually does echo in the books. As spying goes Bond is an amateur.