I don't think he ran away from school at 13 years old to join the Navy, rather his family arranged for him to go to sea on merchant vessels. He sailed on clipper ships, and passed through Tasmania and New Zealand.
Bell actually returned to England some time around 1910 and joined a flying school. In Africa he was inspired by news of a flying race, and came up with the idea of using a flying machine to spot elephant from the air. He wrote to his sisters and asked them to send him out a flying machine and fuel to run it. His family wrote back advising him that apparently flying an aeroplane required 'some tuition'. He came back and joined a flying school in order to learn, but there was hardly any flying done, they mostly just taxied around on the ground and he became disillusioned with it and went back to Africa. This foreshadowed his joining the RFC when the war broke out, and explains how he went straight into flying.
He once shot at a German plane, and his gun jammed after firing a single round. The Hun plane flew off into a cloud. Some people on the ground saw a plane crash that same day and his commanding officer wrote it up as an aerial victory over Bells objections.
He did shoot a French plane down though...it was a special version of a Spad, fitted out for high altitude flying in order to get a German reconnaissance plane. Bell and his friend Wynne-Eyton (who later went with him on his next African safari after the war) both mistook the French plane for the German reconnaissance plane and shot him down. The French pilot survived the crash, and was very pissed, started shooting at them with his pistol when they landed behind him, so they quickly took off again and pretended they didn't know what happened...
He doesn't mention it but he was the first to score an aerial victory in his Squadron, and finished as a Captain and a Flight Commander, with the Military Cross and bar.
He wrote that he had a reoccurrence of his old African illnesses to get over at the end of the war, (no doubt meaning malaria) implying this was the cause of his medical discharge, but in fact his discharge certificate states that he was discharged for 'nervous asthma', which must be interpreted as battle fatigue. It brings the realities of that war a bit closer to home when you realise that this confident youngster who travelled the world; this young man who explored unknown Africa amongst cannibal tribes; who shot a thousand elephant with a deer rifle, was brought to a state where he couldn't sit in an aeroplane without hyperventilating from fear.
