Rick Sternbach’s earliest concept sketches of the USS-Voyager were started in September 1993 during the last season of ST: TNG and DS9.

Many of his sketches were inspired by nature and took the characteristics of the Orca (killer whale), the manta ray, and different birds.

from the Voyager DVD extra’s disk:)
Rick Sternbach: “Voyager was very much an outgrowth of the Galaxy-class Enterprise that we saw in [TNG]. If you look at a lot of the basic features of the hull you had a sort of a blue-grey colour, you had a defensive shield grid over the surface, the phaser strips were there, the transport emitters, the running lights, all the features, all the surface details that a good starship should have were moved over from the Galaxy-class right onto Voyager. Voyager’s big thing was its smaller size and a new shape. Taking the initial vague description of the ship that we got from the producers, “sleeker, faster, bullet-shaped, smaller than the Galaxy-class”, got me to thinking about, you know, some of the shapes and colours we had used in [TNG] and on [DS9] and I began sketching a smaller ship with, not so much an elliptical saucer the way the Enterprise-D had, but something slightly pointier in the front to convey a sense of speed, you know, a smaller troubleshooter spaceship that could go from crisis to crisis, you know, each week. Based on some of the initial sketches that the producers had approved the direction that they thought Voyager should be going in, I was able to put together a first prototype in blueprint form, not the final ship, but a very early prototype shape, slightly angular as compared to what the final ship looks like, and in that initial blueprint view I was able to get the top view, bottom view, side, all of the straight-on plan views that would show them what the ship would look like, along with some perspective sketches. And once that blueprint view was either given a very initial sign-off they said “ok, go ahead and make the mock-up”. Once the mock-up was delivered and the producers had a chance to look it over, Jeri Taylor gave me a quick call and said, “Can you make it more curvier, kind of like a Lexus?” And I thought, “Why not?” The mock-up and the initial blueprints were a jumping-off point. Every major Starfleet ship has gone through piles and piles of sketches and blueprint drawings and preliminary models to get to that final shape that everyone loves.”