Nigel

Per wiki standards - when to use and external link:

 1) Is it proper in the context of the article (useful, tasteful, informative, factual, etc.)? 
 2) Is it a functional link, and likely to continue being a functional link? 
 3) Each link should be considered on its merits, using the following guidelines. As the number of external links in an article grows longer, assessment should become stricter. When in doubt about the appropriateness of adding new links, make a suggestion on the article's talkpage and discuss with other editors.

I generally avoid external links except when it takes me directly to the home page of the aircraft in question - this link does take me directly to NASM's page on this particular P-61 - that makes this external link appropiate.  Agree that this page does need more references.