The 11-year-old was a 25-1 shot after finishing third in the Pertemps Final over hurdles at the Cheltenham Festival and moved into the race travelling powerfully.
He jumped the final fence in a clear lead and his pursuers never looked like bridging the gap as he galloped all the way to the line to secure a momentous five-length victory.
Balthazar King ran a fine race to finish second, ahead of champion jockey Tony McCoy aboard Double Seven.
Alvarado stayed on from a long way back to finish fourth.
Aspell, who at one stage had retired from riding, said: "It's a wonderful day, this is what we do it for.
"I've been watching the National since I was a very young boy.
"As much as you enjoy sharing everyone's success, you crave a bit too.
"To get a chance to ride in the National is a great thing, and to get on one with a chance is even better."
Aspell went on: "I was very conscious that I didn't want to be in front too soon over four and a half miles.
"He's a small horse, so he finds jumping hard enough.
"When I got a bit of daylight I knew I'd be fine because that was my only worry, a lack of daylight.
"Once he was in daylight he really enjoyed that part of the race."
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