Although conceived at the same time as Mariner A, Mariner B was heavier and more ambitious.
The probe was conceived from the outset to be able to explore both Venus and Mars, with the possibility of landing a small capsule on Mars. For this kind of flight the probe was to have flown-by the planet some 15.000 km away while the landing capsule, a possible relative to the Corona spy satellite film re-entry canister, was to experience atmospheric braking before deploying its parachutes in order to make a soft landing on the surface of the planet.
The fly-by bus was to carry a similar instrument complement to Mariner A, with the addition of a TV camera yielding a 1 km ground resolution. The landing capsule was to carry some instruments for atmospheric analysis (barometers, thermometers, a mass spectrometer and a gas cromatographer) and a camera to take pictures of the landing site.
No less than four Mariner B flights were initially planned: two to mars in 1964 and two more to Venus the next year.
The mass of each probe was to be close to 850 kg and the launcher was to be the Atlas-Centaur.
On May 8, 1962, the first Atlas-Centaur was launched from Cape Canaveral. Sixty-three seconds after lift-off the rocket exploded. This led NASA to reschedule the launch of the first Mariner B to Mars to 1966, effectively canceling the program as this launch window was already reserved to the overly ambitious Voyager Mars lander which, in fact, never flew.
As a substitute to Mariner B, JPL proposed launching a couple of simplified Mariner probes to Mars in 1964. These probes, initially known as Mariner C, became Mariner-3 and -4 after launch. During the summer of 1965, Mariner-4 became the first man made probe to take pictures and scientific data of Mars.

BIBLIOGRAPHY
Ezell, E. C., Ezell. L. N. "On Mars", NASA SP-4212, available on-line.

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