not sure what they were working on that prompted them to renew the disinfo program from the 50's...,

wikipedia | The series was produced by Quinn Martin, who was looking for a show to replace the immensely popular The Fugitive, which was ending its run in 1967. Larry Cohen, the show's creator, had conceived two earlier series with similarities to The Invaders. Chuck Connors starred in Branded (1965) as a soldier court-martialed for cowardice, who traveled the West searching for witnesses and proof that he had acted valiantly, and Coronet Blue (1967) about Michael Alden, a man suffering from amnesia who was being pursued by a powerful group of people. All he could remember were the words "Coronet Blue".

Another inspiration was the wave of "alien doppelgänger" films which had come ten years before in the 1950s, typified by Invasion of the Body Snatchers (1956) and the British film Quatermass 2 (1957), known in America as Enemy from Space. While these paranoid tales of extraterrestrials who lived among us, posing as humans while planning a takeover, are usually linked with a Red Scare subtext, Martin simply wanted a premise that would keep the hero moving around and that would explain why he could not go to the authorities (i.e. not only had some aliens infiltrated human institutions already, but most humans would dismiss a claim of alien invasion as a paranoid delusion), however as the series unfolded the various 'disappearances' of people in episodes (killed by The Invaders, such as Vincent's partner - James Daly - in the pilot, etc.), those installed alien figures revealed to be aliens by Vincent thus having to withdraw (such as Edward Andrews' character in 'The Mutation' etc.) plus the surviving one or two key human witnesses in most episodes (from the third episode onwards) did rather alter the basic premise of the show to something deeper and more thought provoking early on.

The basic idea of just ONE man standing between Earth being invaded by an entire alien force with advanced technology, rather stretched viewer credibility (and hardly made the aliens themselves look very impressive as Vincent consistently turned up everywhere and defeated them each week), the episodes do however have curious undercurrents both re the political overtones and quite subtle hints that more was going on than at first appeared, making it a far more compelling show than it first seems, anticipating later such shows as' The X Files' and 'Dark Skies' etc., which were clearly influenced to a degree by 'The Invaders'.

The flying saucer design was influenced by two famous UFO photographs. The first case happened in 1965 in Santa Ana, California. On August 3, the highway traffic engineer Rex Heflin took several pictures of a flying craft, while working near the Santa Ana freeway. Heflin did not report his sighting, but the photographs were published by the Santa Ana Register on September 20, 1965. The second is the Adamski case. On December 13, 1952 in Palomar Gardens, California, USA, the contactee George Adamski took a series of photographs through his telescope, of a bell-shaped craft, today well known as the Adamski Scout Ship. The upper hull, and flat top from the Heflin case were combined with the bell-shaped outer flange and three rings of the Adamski case. The five hemispheres in the bottom of the craft seem to emulate the three semispheres in the Adamski Scout Ship.