The basic concept of Roald Dahl's Going Solo is to continue the autobiographical narrative that Dahl starts in the publication of his first autobiography titled Boy.
In the book Boy, Roald Dahl makes an account of his childhood whereas in Going Solo focuses on all the different careers in which he engages which include serving in the Royal Air Force, as a worker in an oil company, and as part of the War effort in during the German invasion.
Basically, Going Solo is a recollection of most memorable moments in his adult life. In comparison to Boy, Going Solo is a thicker and more complicated read, as it deals with topics that are geared toward an older mature audience. And, to answer your question, it would be considered non-fiction as it is a memoir of things that actually took place in Roald Dahl's life.