"When I turned back towards the house, I saw that a window had been pulled out from its frame. I could hardly breathe from fear. I retreated inside - a torch was shone in my face.
Mr Martin, 55, spoke of his feelings on the day after his solicitor launched an appeal against his life sentence.
In an interview with the Daily Mail he revealed: "I didn't intend to kill anyone. I didn't think about shooting anyone in the back, legs or anywhere else.
"I didn't actually see anyone at all. I was terrified. My mind went blank and my actions were purely spontaneous."
His case has provoked widespread debate over issues of self-defence and rural policing.
Speaking from Bullingdon Prison near Bicester, Oxfordshire, Martin said he was coping well with prison life.
"I'm trying to take one day at a time and not thinking about my appeal. At the moment I don't even feel depressed. The support from the public has helped me 101%."
Describing the events on the night of the shooting, he said: "It was pitch black. I couldn't see anyone, but I could feel their presence.
"When I turned back towards the house, I saw that a window had been pulled out from its frame. I could hardly breathe from fear. I retreated inside - a
torch was shone in my face.
"It blinded me instantly and instinctively I shot from the stairs. I can't remember how many times I fired, but I know I aimed down. Of course, it's regrettable that that boy died."
He said he passed his days in prison listening to the radio, watching television and reading newspapers.
He added: "The warders are treating me well and several have said that they support what I did. The only appalling thing is that they wouldn't let me bring my teddy bear.
"They seem to think it could be stuffed full of drugs. Otherwise I'm nowhere near as frightened of my life now as I was when I was living in Bleak House."
Martin's solicitor Nick Makin said on Friday that one of the grounds of appeal was that "a fair trial was not possible in an atmosphere of intimidation of both witnesses and jury".