The character Conan the Barbarian AKA Conan the Cimmerian was first introduced by Robert E. Howard in 1932 adding 21 stories to the genre before his death. Conan did not die with Howard several authors have continued to publish new stories with
Boris's artwork gracing their book covers. Indeed Boris began his career creating comic covers for Savage Sword Of Conan. Below is an incomplete selection of artworks by Boris and Julie featuring Conan.

Doc Savage (Clark Savage Jr) is a fictional character from pulp fiction comics of the 1930's and 1940's. Created by publisher Henry W. Ralston and editor John L. Nanovic at Street & Smith Publications. The character is portrayed as derring-do, multi-faceted, righter-of-wrongs.

Boris Vallejo's involvment with the Doc Savage character began with a commission for six book covers (books #83 to #88) from Bantam Books who had been reprinting all 190 of the Doc Savage novels since 1964

In 1934 King Features Syndicate required a comic-strip space-hero to compete with the already succesful Buck Rogers. After failing to secure the rights to John Carter of Mars from Edgar Rice Burroughs they called upon Alex Raymond to create a new character Flash Gordon.
In 1980, Tempo Books released a series of six novels written by David Hagberg. Four of the book covers were painted by Boris Vallejo (shown below) they are Massacre in the 22nd Century, War of the Citadels, Crisis on Citadel II, Forces from the Federation. I believe Ken Kelly painted the two remaining covers, Citadels under Attack, and Citadels on Earth.

John Carter and Dejah Thoris are Edgar Rice Burroughs characters from the John Carter of Mars series of novels. The protaganists are located on Mars aka Barsoom. Carter, being the strong rugged type saves the planet from it's evil oppressors, whilst simltaneously winning the hand of the Princess of Mars Dejah Thoris. Below are four pieces by Boris and Julie inspired by the characters.

Created by author Roy Thomas and first drawn by Barry Windsor-Smith, Red Sonja first appeared in 1973 for Marvel Comics. She is a fictional sword and sorcery comic-book superheroine, partially inspired by Robert E. Howard's character Red Sonya of Rogatino. Boris's involvment with the charcter begins in 1981 when commissioned by Ace Books for six book-covers, #1 The Ring of Ikribu, #2 Demon Night (1982), #3 When Hell Laughs (1982), #4 Endithor's Daughter (1982), #5 Against the Prince of Hell (1983), and #6 Star of Doom (1983).

Created by Edgar Rice Burroughs, Tarzan first appeared in the 1912 novel Tarzan of the Apes (followed by 23 sequels). The enduring character has re-appeared in innumerable works and other media's.
Commissioned by Bantam Books for twelve book cover cover paintings, Boris went on to produce a further two in order to complete the 1978 Tarzan Calendar. Since then, both Boris and Julie have produced several artworks inspired by the character.