There was a saying within the Royal Air Force between the wars was that ‘the bomber would always
get through’…a philosophy that would receive a rude awakening during the second world war.
But until then, this was then this was generally held belief.
And so during the 1920s and 30s the air ministry experimented with all sorts of aircraft designs
in the search for the perfect bomber hunter. Many of these designs involved novel armament
layouts, often including fairly heavy guns for a fighter aircraft.
Some of these designs were more…interesting than others, but they all had one universal drawback:
none of the ministry specifications written for these ‘bomber hunters’
foresaw the need for defending against escort fighters, and so performance and
manoeuvrability were not seen as an important factor for the requirement. As a result of this,
many of these experimental aircraft were designed purely as flying weapon platforms,
and the idea of them having to defend themselves against fighter aircraft was not even considered.
All of this meant that many of these so-called bomber hunters were slow,
cumbersome, and fairly useless, but one of the few examples that could have broken this trend
was the Boulton Paul P.31 Bittern. The bittern was built to meet air ministry
specification 27/24. This outlined the requirement for an interceptor and night fighter that was
capable of breaking up formations of heavy bombers. During a time when the single-engine
biplane ruled supreme, Boulton Paul designer John North took the adventurous step of drawing
up a design that would not be considered commonplace for another decade to come.
His aircraft, the P.31, was a single-seat, twin-engine, shoulder-mounted monoplane – a rapid
departure from most designs of the time. But that being said, North also did his best to keep the
design of the structure as simple as possible to keep costs down and make it easy to maintain. The
fuselage was a basic box-girder structure, without rounded decking or fairings, and covered in
fabric. The wings were parallel chord throughout, the control surfaces were basic square-cut shapes,
and all of these too were fabric covered. Mid-mounted on these wings were a pair of 230hp,
7-cylinder, Armstrong-Siddely Lynx radials, and their nacelles formed one of the connecting points
for the Bitterns wide-track undercarriage. Its design was relatively simple: a single
ole-pneumatic strut, which was supported by V-struts, and its wide track was chosen to improve
the aircraft’s ability to land safely at night. The pilot’s cockpit was located in line with
the wing leading edge, which provided excellent visibility. Not only was this ideal for take-off
and landing, which at night would be aided by the illumination of flares, but it also gave
the pilot an excellent field of view in which to employ the P.31s unique weapon system.
It was designed to have a pair of streamlined barbettes installed on the sides of the fuselage
below the cockpit, and in these would be a single 303 caliber Lewis gun. These barbettes
would be controlled by the pilot, allowing him to adjust the firing angle from straight ahead
up to an elevation of 45 degrees. Aiming the weapons was done through a ring-and-bead sight
that elevated along a special frame to match the elevation of the guns.
This would allow the Bittern to both attack bombers directly in its line of sight, or,
more favourably, it would allow an attack from below where the
bombers were considered most vulnerable. Two prototypes of the Bittern were produced.
The first was completed without the rotating barbettes, the machine guns being installed
in a fixed firing position, and its unknown whether this was due to issues developing them,
or if they wanted a more simple prototype to begin with for airframe testing.
Whatever the reason, this first prototype took off for the first time in February of 1927,
and during its first flight things immediately started to go wrong. Test pilot C.A.Rea noted
an alarming lack of lateral control, but at first they couldn’t figure out why.
The ailerons appeared to have an adequate amount of movement, and ground testing
confirmed no loss of motion between the control input and the control surfaces.
Rea felt he could solve the problem if he were able to observe the control surfaces in flight,
but the ailerons on the outer wing were shielded from view by the engines, even with his seat
raised to the highest possible position it was impossible to see what was going on.
But in typical English fashion a solution was found in the form of the thickest,
stoutest cushion they could find, which allowed Rea to thrust his head uncomfortably high above
the protection of his windscreen to see. During his next test flight, his face being
battered the wind the entire time, Rea noted with alarm that the outer wings were flexing
to such an extent during flight that it was cancelling out the effect of the ailerons.
After easing the semi-controllable plane back to terra-firma, further
ground tests were done to simulate the effects of aerodynamic forces during flight, and it was
found that the flexing of the wings occurred throughout the majority of the speed range.
Along with the control problems making it almost unflyable at anything above take-off speed,
there were also concerns raised about the airspeeds in general. During the initial flights,
wobbly wings not withstanding, it barely managed a top speed of 145mph. This meant that the Bittern
could only catch up to the heaviest, slowest bombers currently fielded, anything in the light
or medium range would be able to match or even outpace it, and with the way things were advancing
heavy bombers wouldn’t be far behind either. As a result of all this, numerous changes were
made to the second prototype, which was then still under construction.
Additional V-struts were added to the outer wings to solve the flexing issue. Along with this the
overall wingspan was increased by five and a half feet, and Handley-Page leading-edge-slats
were installed to improve its handling. The engines were remounted, so that they
sat in nacelles that were slung under the wing rather than within it, and this allowed
the undercarriage struts to be shortened and strengthened. The engines also had new
townend rings installed in the hopes of getting some more speed out of the airframe.
Though it would have seemed an obvious area for improvement, the engines were not changed out for
more powerful models, despite examples with at least double the power output being available.
Some have argued that John North made this decision because adding more powerful engines
would have required a stronger airframe, and thus a heavier airframe. However, had the correct
engine been chosen – and for this a Bristol Jupiter would have been ideal – the increased
power would have negated the drawback of increased weight and at the same time there would have been
enough power left over to increase the top speed. But this of course was never done, and so when the
second prototype took flight its top speed was only increased by about 7 mph. That being said
it was at least stable in flight, and between 1927 and 1930 it completed various trials at
Martlesham Heath. As it was only slightly larger than contemporary biplane fighters of the time,
its handling was quite good. Indeed, for a monoplane it impressed many people in this
regard- and we must remember that during this time there was still a strong bias against monoplanes
in Britain, misguided though they were. Also, unlike the first prototype,
this second model did have the rotating barbettes installed for the machine guns,
and these too impressed evaluators. But speed was the big elephant in the room,
and it was generally accepted that the Boulton Paul Bittern was too slow,
a point highlighted by the fact that Boulton Paul Sidestrand bombers could match its top speed, and
the newly designed Bristol Bulldog fighters could easily outpace it when it came to bomber hunting.
As a result the aircraft was not ordered into production, and the two prototypes
would remain as the only examples built. Both aircraft continued to fly into the early 1930s,
being used for various tests, particularly around their wings, but at some point, around the middle
of the decade, they were most likely scrapped. Had the Bittern been built with more powerful
engines from the outset, and thus been faster, it could have revolutionised the design of
British night fighters and interceptors. But as it was, they were a failure,
and by the mid-1930s the RAF had moved away from the idea of a dedicated night-fighter,
instead calling for service aircraft that were equally suitable for both day and night
operations. This would lead to numerous designs, chief among them being the Hawker hurricane. But
that’s a much longer story for another day. As always thank you all so much for watching,
and a big thank you of course to the Patreons, with a special shoutout to Kevin,
Delliardo, Bane, FB, Christopher R, Tronathon, Erik Hindman, and John Austin Jr for their support
as wing commander tier patrons. Thank you all so much, and I’ll catch you all next time.