The Skywarrior bombers had a self-boarding arrangement* that also did double duty as a bailout slide:
It consisted of an inner (upper) and outer (lower) door. When closed, the inner door formed the bottom of the cockpit floor and sealed the crew compartment for pressurization; the outer door closed off the opening in the bottom of the fuselage. Large indentations, two in the inner door and three in the outer door, served as steps and hand grips and still allowed the doors to have a smooth surface to function as a slide for bailout.
However, the following picture of a crewman ascending into the cockpit included a feature not included in the illustration above, a rectangular transverse ledge (highlighted by question marks) with a raised non-skid pattern.
On close examination (you can see successive ledges below the top one), I finally realized that in this instance, climbing into the airplane had been made easier—particularly if you had something in your hands—by leaning a folding ladder up against the outer door.
* Skywarrior versions had similar arrangement that was different in detail, including the arrangement of steps: