An ill-timed power failure made hash of my post. I’ll try again.
The B-24 and B25 had twin vertical stabilizers located directly behind the engines. The planes worked well enough, but I know of no other use of this design.
Was the design not very good? Did it not work well with jet engines?
I was thinking of this because of the crash that occurred when a vertical stabilizer broke off an Airbus.
There are a number of other aircraft designs with double, even triple tails, and not necessarily with multiple engines either. As you suspect, the main advantage with the Liberator and Mitchell was having the rudders in line with the high-velocity propwash, making them more effective. There was also the benefit of some redundancy, in case one got shot off, and the tailgunner’s compartment could be larger and more user-friendly. On the down side, the horizontal stabilizers had to be much stronger and heavier to take the vertical stabilizers’ loads as well as their own.
The Lockheed Constellation’s triple tail, according to legend, was put there at TWA’s behest so it could be lower than a single or double tail, and could fit into the door of their main maintenance hangar in Kansas City.
The only multi-tail jet I know of that was designed that way, instead of having more surface added to a derivative of an existing design for some reason, is the A-10. The vertical tails not only provide battle-damage redundancy, but help shield the jet exhaust from heat-seeking missiles.
Normally, yes, you’d design and build the tail to be unbreakable in normal usage, although there are extreme exceptions. Something was overlooked in the A300, and figuring out what it is in a composite structure rather than a metallic one is proving difficult.
Many modern fighter aircraft have twin vertical stabilisers (fins). Think F-14, F-15, F-18, F-22 and the new Joint Strike Fighter. They’re not used for redundancy, if that’s what you’re thinking. (I think the A-10 is the only modern aircraft reported to be controllable with one of its two fins removed.) They’re used because for the tremendous directional control forces required on these aircraft, a single tail would be unreasonably large.
Then again, the F-16 has a single fin. Twin fins are more complex, heavy and expensive to build than a single fin, so it’s a trade-off. Like everything in aircraft design.
Because of the tapered-tube shape of modern large commercial transports, the simplest and most efficient tail design is the current single-fin cruciform shape. it would be very difficult to mount twin fins back there. They would have to be stuck on the ends of the horizontal stabiliser, which would have to be strengthened significantly. This would impose a large weight and performance penalty on the aircraft.
The Airbus A300 fin is already a fail-safe design. (It’s attached to the aircraft with six lugs. One lug can be lost without affecting controllability.) Unfortunatley, as ElvisL1ves said, the crash may have resulted from an unforseen design deficiency. This does not imply that the single-fin design, metal or composite, is inherently deficient.
Incidentally, Boeing’s new Sonic Cruiser, a commercial airliner meant to carry 200 people at nearly the speed of sound (now in development), may have twin fins. The aircraft is a delta-wing shape, so the fins could be attached without adding much support structure.
Twin tails aren’t unusual, really. Besides the B-24 and B-25, the Lockheed P-38 had twin tails of a similar design, as did the British Lancaster bomber, the Beechcraft Bonanza, and the Lockheed Electra. The Lockheed Constellation had a triple tail, as mentioned before.
The A-10, also mentioned before, is a jet design with a tail design very similar to that of the B-25 and B-24. Other twin-tailed jet designs include the F-14, F-15, F/A-18, MiG-25, MiG-29, Su-27, Su-37, F-117, SR-71, and many others that I have left out.
Your question seemed to ask if twin tails in general were a rare feature in modern aircraft. No, they’re not. The specific arrangement of the tail surfaces of the B-24 and B-25 is not unusual either. Tail designs are just like the rest of the aircraft - whatever design best fits the needs and purpose of that aircraft is the design that is used. Maybe modern airliners don’t have twin tails simply because they don’t offer enough advantages to offset the extra weight they would add to the airframe. Not being an engineer, I can’t say, but being an airplane fanatic, I can throw plenty of examples at you.
Hmmm…seems that Kamandi and I were typing at the exact same time, saying basically the same thing. So you know we’re right.
Let me see if I can add anything (dusting off the old “Field guide to airplanes”)
I’d have ruled out the F-15, F-18, and SR-71 type tails as “similar” to the B-24. Twins, yes, but they’re not attached to the ends of the horizontal stabilizers, and these aircraft don’t have a tailboom to speak of. I’d also discount the P-38, OV-10 and Cessna Skymaster types, because they have twin tail booms.
H-tailed military aircraft that come to mind(do the google search yourself for photos): A-10, T-46 a jet trainer that didn’t enter production, and the C-23 Shorts (Sherpa?)
Civilians: Beech 18, Ercoupe, and the Shorts derivatives. Those Shorts airplanes are the ugliest flying boxes I’ve ever seen.
More than two fins: OV-1 Mohawk, E-2 (Naval radar plane) and the aforementioned Constellation.
Good things about multiple tailfins: lower control loads (means less pedal forces in unboosted airplanes) and less vertical space needed, some redundancy.
At first I thought that having the rudder in the slipstream was a good thing too, but after thinking a little, I kind of doubt it. In a low speed single engine out situation, you’re likely to have so much sideslip oscillation that I bet the fin would move into and out of the prop blast, making yaw control more difficult.
Bad things: heavier structure needed to carry the loads, more contol system complexity.
Nowadays, I doubt anything would justify the weight except the “envelope” considerations (like the E-2) and survivability (A-10).
I’ve seen some endplates put on the ends of helicopter horizontal stabilizers (not movable), but it’s always been a band-aid solution to not having enough tail to start with. That’s why they’re being tried on the RAH-66 Comanche.
I’m inclined to doubt the lower control loads. The rudder area that has to be moved is whatever is required for proper control under various conditions, like engine-out. That should be the close to the same number independent of the number of vertical control elements. Tip effect would be more prominent in the case of the short fins in the dual case, but otherwise the two setups should be equivalent.
I also doubt the “rudder in the prop-wash” thing. I’m not sure that’s a good thing.
Incidently, Consolidated made a single-vertical-fin version of the B-24 for the Navy’s use as a patrol aricraft. I’ve forgotten the designation and I can’t find it on the net. It had a single large vertical fin and rudder.
PB4Y-2 Privateer
Here is a link to information on the Consolidated Aircraft patrol bomber, PB4-Y. http://www.ixpres.com/ag1caf/usplanes/aircraft/privater.htm
Concerning the F-14, F-15, F-18 - these are all twin jet designs, withe the engines in the fuselage, so the fuselage is (relatively) wide and has a fair amount of room available. The F-16 has one engine in the fuselage, so the fuselage cross - section is more nearly circular. I don’t suppose there’s much point in putting the verticals 2 foot apart, and I’d bet there’d be airflow problems if you did. At any rate, since they had the room, they used two fins.
This doesn’t apply to airliners/cargo planes, as they hang their engines on the wings.
Darn! When I click of the link I get a blank screen. Closing that screen brings up the screen for the PB4-Y. BTSOOM why that happens.
Re: BTSOOM
This should go into the acronym FAQ, IMHO
Russia has some twin tail beasts worth mentioning (from my favorite Russian aircraft site):
The An-225 “Buran”, the An-38/An-38-100, the An-28, the An-22, and the An-14 (which looks like it’d be a useful puddle-jumper). Does anyone else get the idea that Antonov liked twin-tail designs?
Sukhoj OKB does twin-tails, also, but this is a more conservative twin-boom/twin tail design: S-80.
The SAM-7 project is very interesting, with it’s tail-less twin stabilizer design. It actually flew, and was found to be stable, but was cancelled (most likely due to political infighting).
The SBB, designed by Arhangelsky, is contemporary with the Liberator, but never reached anything like similar performance.
Here’s a curiosity: A turbo-prop biplane, first flown in 1984! The An-3.
Enough. Prowl around the site on your own… There’s plenty to read, and Alexandre Savine deserves much credit for such an excellent site.
That’s a good point. On those twin-engined aircraft most of the structure required to carry two fins is already in place. Putting two fins on an F-16 would require a significant weight increase.
Take a look at the Panavia Tornado. It has two engines (although they’re mounted close together) and a single tail. But look at the size of that fin! It’s huge! The designers could have put two fins on it, outboard of the engines, but they decided to go with one. Why? I dunno. I guess the design tradeoffs favored a single fin in this case. Looks weird, though.
I’ll second that. 
Actually, I was waiting for someone to mention the F-111, another two engine fighter with a huge vertical fin.
BTSOOM. 
I like it, daddy. Can we keep it?
A friend who used to work at Grumman Bethpage said the design concept for became the F-14 called for a huge vertical fin a la F-111 but was switched to a twin so it could be parked in the basement on a carrier. Hinges fins like the S-3 viking use are an alternative but add weight and complexity.
On a side note, the tomcat caricature used in many F-14 squadron insignia is distinct from other scrappy tabby toms in that he has two tails and wears a holstered pistol on his left side, where the F-14’s M61 vulcan is mounted.
Of course there was also the Rockwell A-5 Vigilante, which had twin jet engines and a single vertical tail. (The MiG-35 Foxbat is said to have taken some design cues from the A-5.)
I wear my Colt Navy on the left. Crossdraw. (But the revolver on the patch has the butt back for a left-handed draw.)
Nitpick: The tail configuration you’re writing about is called a “conventional” tail, I think. A cruciform tail has the horizontal stabilizer about half way up between the fuselage and the top of the tail. A T-tail has the horizontal stab on the top.
The Bonanza had a V-tail. The design was eventually abandoned in favour of a conventional tail.
Of course when I fly, the tail goes round and round very fast. 
(There is a single oblique surface at the tail of a 300CB and a |- shaped surface at the end of an R-22.)
Another disadvantage of multiple tail surfaces is an increase in intersection drag. All else being the same, you’ll get better aerodynamic performance out of a single tail.
It’s rare but there are tails with a curciform configuration with the intersection at the fuselage, an equal amount of vertical fin underneath. Most missles I know of have this cruciform configuration (though they all fly in St. Andrew’s Cross mode :D)
We’ll have to teach you to shoot left handed son. I’ve been shooting with both strong and weak hands since I started CAS with SAAs and now navies. FWIW the butt forward carry doesn’t always mean crossdraw. Wild Bill carried his navies butt forward in a sash but used a cavalry twist draw, starting with palms out and twisting the gun around after it comes out of leather. My compadre Canada Bill draws his '60s this way as it’s easier to clear the long barrels from the high riding slim jim holsters.
Actually, a cruciform tail has the horizontal stab. above the fuselage and below the top ot the vertical stab.
I was going to mention Wild Bill, but I decided not to. I’m a fair shot with my left hand. When I go to the indoor range (with smokeless pistols) I switch off, shooting one magazine with my left hand and one with my right, and so forth. When I went out shooting my Navies I wore two holsters butt-forward. I’d crossdraw when they were more in front and twist-draw when I had them farther back. Personally I liked the crossdraw better.