Effects of Bombing at Myitkyina

HEADQUARTERS TENTH AIR FORCE

U. S. ARMY

Office of the Ordnance Officer

APO # 487

17 AUGUST 1944

REPORT OF EFFECTS OF AIR SUPPORT AT MYITKYINA

I. GENERAL:

1. Purpose of this report is to collect from all possible sources information on effect of air attacks at Myitkyina in order to determine size and type of material to be used in future attacks on Burmese towns.

2.  Sources of information are as follows:

a.  The best source of information on effect of bombing was from the U>S> Liaison Officers on duty with the Chinese Army.  This is because the opinion of the observer is focused on the vital question of close ground support, whether the attacks facilitate the advance.  Also, before a defended point is charged, it is subjected to much gun and mortar fire so that the physical effect of the air attack is difficult to ascertain.  Also, evaluation of damage is difficult after the assault because positions containing enemy dead have to be covered with soil to reduce odor.  A consolidated report of liaison officer’s observations is being forwarded through channels by the Task Force Commander.  The report of Lt. Col. Orth, Chief Liaison Officer of the 150th Regiment is attached as inclosure number 1.  The author visited the battlefields with this officer who pointed out the effects of our bombs on various types of positions.

b.  Aerial Photography is useful for evaluation of structural damage, but bunkers were usually so camouflaged or covered with debris that damage to the underground structure could not be ascertained.  Inclosure number 2 shows a vertical before, and an oblique after a hit with a 1000-lb. Mark 13 mine used as a bomb.

c.  In order to get a summary of Prisoner of War information, a conference was held with the interpreters employed in P>O>W> interrogation.  Information secured from that source should be considered with reserve since usually the prisoner of war’s opinion of wha is most effective is limited to what he actually experienced and in most cases will naturally be the item which eventually led to his capture.

II. DESCRIPTION OF MYITKYINA DEFENSE.

1.  In general, three types of occupied positions were encountered.  We may call these heavy, medium and light depending on the amount of construction involved.

a.  The heavy positions were usually under two story frame buildings.  The roofs of these buildings were either very thin sheet iron or flat red tiles.  The sides were usually of thin shingles although a few two-course brick walls were present.  All framing was very rugged except roofs which were comparatively fragile.  Connection between frame members was comparatively weak.  The outstanding characteristic of these houses was a foot-thick concrete ground floor usually about a foot above surrounding ground level.  The Japanese employed this concrete floor as their main protection from bombs.  They dug under this, put two crossed layers of six inch legs under it and put up six inch square legs  as shoring to support the dirt walls.  Fire ports were made by breaking out a small section of the foundation walls of the houses.  Usually three sides of the house were protected by these dugouts, the fourth side being the only one on which an assault could be made.  To improve the resistance of the concrete floor, lumps of  concrete were piled two feet deep over the dugout area and on top of this three feet of dirt and sod were piled.  Inclosure number 3 shows a heavy structure.

b.  Medium positions were constructed in the less substantial buildings that did not have concrete floors and foundations.  The roofs were made of crossed layers of 6” x 6” logs on which were placed several courses of bricks, a layer of sheet metal and then several courses of sand bags on top of which was placed several feet of earth and sod.  Inclosure number 3 shows such a structure.  Photograph in inclosure 4 shows nature of building employed for this type of position.

c.  As soon as time was available, any spot suitable for defense was covered with a hasty entrenchment consisting of 8 inch uprights covered by 8 inch logs on which 2 to 3 feet of earth were placed.  These positions were placed as close to a large tree as possible to conceal and to protect it from air attack.

d.  In railroad beds and highway fills transverse tunnels were dug and machine guns placed facing toward our lines.  Personnel on the far side of the railroad bed were comparatively safe from heavy machine gun fire.  Where rail wagons were available they were filled with dirt, ground or wood, and tunnels dug under them to protect men from bombing.

e.  Underground storage areas were made by digging pits 5 feet deep, 10 feet by 30 feet in length with entrances at each end.  Inside walls were protected by log shoring and angle iron (4” c 4” x 3/8”) was laid across the narrow dimension, on the angle iron logs were laid.  This was covered with corrupted sheet iron and then 5 feet of earth and sod.  No hits or near misses were seen on this type structure.

III. TACTICS.

1. Fighter:

a.  In demolition bomb attacks, if ceilings permitted, dive was from 6000 feet, 15º to 45º dive angle, speeds varying from 225 m.p.h. in case of P-40, to 325 m.p.h. in case of P-51.  Altitude of release was around 1000 feet.  In many cases low ceiling necessitated shallow glide angles using 8-11 second delay fuses.  In this case release was as low as pullout would permit.  Inclosure number 5 shows tonnage dropped on Myitkyina by fighters and fighter-bombers.

b.  75-gallon belly tank incendiaries were used against strongpoints.  The tanks used were of the standard P-40 type filled with 70 gallons of an incendiary mixture of fuel oil, volatile spirits and a small amount of mixed lubricating oils, paraffin wax and asphalt base and topped with 5 gallons of aviation gasoline to increase inflammability.  Two M-4 igniters were wired to the parallel plates that hold the suspension lugs  of the tank.  These were all dropped from P-40N aircraft at an altitude of release of 25 to 500 feet, with indicated air speeds of 220-300 m.p.h., dive angles from 0º to 45º.  The belly tanks were sometimes dropped alone, at other times they were released at the same time as two 100-lb. M47A2 Napalm bombs carried on wing racks.  This combination gave higher percentage of ignition.  Napalm solution is not available to this air force for belly tank filling.  Within the range of altitudes tested, altitude did not seem to be important.  Tanks released at too steep a dive angle buried into the ground with result that oil did not spread, while those tanks released in horizontal flight spread the oil over too large an area and did not ignite.  The best angle of release was found to be between 30º and 45º from an initial altitude sufficient to attain an indicated air speed of 250 m.p.h. at time of release.  This could be done from 1000 to 1500 feet, which is important because greater ceilings are not found during the monsoon season.  About 80 tanks were dropped between 25 July and 3 August.

2.  Medium Bomber:

Medium bombing was done from 5000 feet where visibility permitted, in some instances bombing as low as 300 feet was necessary.  Attacks were made in flight or squadron formation.  Inclosure number 5 is a preliminary statistical summary of tonnage dropped on the town of Myitkyina during the engagement.

IV. RESULTS OF ATTACKS.

1.  Aerial photography does not reveal whether a defensive position under a house has bee destroyed.  Inclosure number 2 shows a vertical of a two story house before and after a hit by a 1000-pound Mark 13 Mine.

2.  Effects of Bombs.

Reports from ground sources and actual inspection of the town after the end of resistance is summarized below.  In this discussion a near miss is considered to be a strike within 10 feet of the target.  Results obtained may be generalized as follows:

a.  100-pound demolition bomb.

(1)  Against light and medium structures and trench systems, direct hit is effective, no near miss damage.

(2)  Against heavy positions direct hit is ineffective.

(3)  Against residential buildings a direct hit would blow off roof, side sheathing, a section of flooring without causing any outside wall frame failure.

(4)  The number of 100-lb. demolition bombs employed was so small, no hits could be identified on rail cars or bashas.

b.  250-pound demolition bomb.

(1)  Light and medium structures and trenches - direct hit destroys - year miss causes severe damage.

(2)  Heavy structures - direct hit destroys unless it is under a building, in which case the bomb destroys the building but does not penetrate the concrete bunker.  A near miss will cave in a side of the structure.

(3)  High grade residential buildings are destroyed by direct hit, near miss cause sheathing and roof damage but not collapse.

(4)  Bashas are demolished by direct hit, collapse on side toward a near miss.

(5)  Railway boxcars - direct hit destroyed superstructure of cars and part of flooring and undercarriage.  No damage to dug-in positions under cars suffering direct hits.  Near miss damage to steel cars was negligible, moderate on wood cars.

c.  500-pound demolition bomb.

(1)  Light and medium structures and trenches - destroyed by direct hit and near miss.  Slit trench system 60 feet long was obliterated by hits.

(2)  Heavy structures - destroyed by direct hit or close near miss even under building.

(3)  Residential buildings - destroyed by direct hit, half of building collapsed by near miss.

(4)  Railcars - direct hit destroyed, near miss over-turns and severely damages.

d.  Fire bobs and incendiaries.

A detailed study of each strike using the belly tank incendiary has been made by the Chemical Warfare Section.  All buildings which were burnt were very thoroughly destroyed and little debris was left.  In general, structures were of wood and easily set afire.  No dugouts which were hit by belly tanks were seen.

e.  1000-pound demolition bomb.

Effects of thousand pound bombs were not much greater than those of 500-pound bombs.  Very few hits were obtained in the town area with this size bomb.  No direct hits could be identified.

Survey of the battlefield revealed the following facts

a.  The most vulnerable portion of the prepared dugouts were the entrances and fire ports.  Most bomb damage occurred here.

b.  Direct hits were rare in proportion to the near misses.

c.  Ground was very soft and damp, so that very large craters were formed.

d.  Damage above the ground is a great deal easier to inflict than that below the ground.

e.  Camouflage was superb and target identification possible only at very short range.

f.  Destruction of unoccupied building in the interior of the lines helped the defense by providing a source of building materials and piles of lumber.  Wrecked buildings were used as positions concealing the enemy.  When a building was destroyed over him he just cleared his field of fire and stayed put.

3.  Effects of Incendiary Belly Tanks

a.  If the target had substance enough to stop the bomb and was made of wood, a satisfactory fire resulted.  If the target was made of wood but had been so knocked about the bomb could pass through it, the incendiary effect was small since the fire burned out very quickly.  When used against a target made of wood and brick the results were not very satisfactory.

b.  Against open trenches and fox hols the belly tanks were effective in flushing the enemy from them and thereby exposing him to machine gun and rifle fire.  When used against machine gun positions under trees and bamboo, the results were equally good.  However, if such positions were dug in with an earth roof over the firing slit or if they were pillboxes and well-covered, they could not be knocked out by belly tanks.  Ground troops repeatedly reported continued fire from such enemy positions after our fighter-bombers had passed over.  It was originally believed that the occupants would be injured by the initial flash of the bomb or suffocated by exhaustion of oxygen in the pillboxes, but the openings were apparently too small to permit such results.

c.  The fire bombs took the Japanese completely by surprise and probably had a big psychological effect for seven days after their use began the Japanese were driven out of Myitkyina.

d.  The area of fire produced by belly tank with two M47A2 bombs was 20’ x 40’.  Detailed estimates of damage done by incendiaries obtained from aircrews is shown in inclosure number 6.  Results of inspection of incendiary targets after occupation of the town are shown in inclosure number 7.

4.  Results of Prisoner of War Interrogation.

The interpreters gave the following as the consensus of opinion of all prisoners examined to date:

a.  More terror was created by B-25’s than by fighter attacks.

b.  Casualties in dugouts were small.

c.  Most bomb casualties were from flying bomb fragments.

d.  Long delay bombs and duds disrupted enemy activities when any unexploded bomb was found.  The area was evacuated for 800 feet until a 12 hour period had elapsed.  Presence of any unexploded bomb caused very great tension after our first use of long delay fuses. 

e.  The fire bombs frightened the enemy, but casualties were few.  However, the number of fire bombs dropped was so small that very few were in contact with them.

f.  Strafing was little feared because it causes few casualties.

V. DISCUSSION:

1.  The very small size of a dugout makes a hit very improbable.  For example, to score a hit on a 20’ x 40’ building is quite a feat.  Actually the vulnerable area under the house is one-fourth of that area.  The main bomb damage was done by very near misses.

2.  The problem of penetration of bombs exists only in the case of the heavy positions.  In these positions a bomb larger than 100-lb. is necessary to give the bomb time to penetrate.  Actually the comb comes to rest very rapidly and it is a mere question of energy for penetration.  However, the 8 to 11 second delay fuze is much favored because of the greater accuracy attained when the fighter-bombers come in close

3.  The setting of .1 second fuzing on any bombing is excessive because the greater part of the energy and fragments of near misses is wasted.  A fuze setting of .01 second will allow all necessary penetration into all but heavy structures and at the same time keep the explosion sufficiently high to get a good spray of fragments into fire ports and adjacent slit trenches.  Short setting would give shallower craters.  The roofs of even the best construction will many times not cause our M103 and M100 series fuzes to function.  Evidence of the efficiency of the shorter time delay fusing is the photo of the 1000 lb. mine effect which was set for instantaneous action using a Mark 19 Navy fuze which is even quicker than the M103 set “instantaneous”.

4.  There is a psychological factor in close support which should not be neglected.  The loud report of a large bomb, especially one going off above ground, gives the troops the feeling that the enemy is knocked out, and hence gives them courage for a charge.

5.  The recoverability of these positions is very great.  The effectiveness of close support, therefore, depends on immediate exploitation of damage and the shock effects.  The effect of bombardment of battalion or regimental lines is, except for casualties to personnel, remote and unless an immediate break-through occurs the rear positions will remove completely.

6.  In connection with the choice of bomb, the supply factor should be considered.  Effective near miss damage is very limited in distance, but does increase with the size of the bomb.  However, near miss damage from even a 550-pound bomb does not exceed five feet.  However, this distance does increase effectiveness materially on very small targets.

7.  The use of long delay bombs is very effective where used in limited quantities.  The use of long delays immediately causes fear of all duds, thus multiplying the effect of the fuzes.  The effectiveness could be increased by lengthening the time delays so that areas in enemy held lines could be restricted for a longer period.

8.  The Mark 13 Mine used as a bomb was effective.  However, the 500 lb. demolition bomb would undoubtedly have achieved the same effect.

9.  The incendiary belly tank, from reports, has proven to be successful, when used against personnel, however the incendiary tank does not have the necessary pay load per plane value, nor can it be utilized to the best advantage when used as an incendiary for the sole purpose of destroying materiel as compared with smaller and more easily carried incendiary units with better ballistic characteristics and wider dispersion possibilities, and which are available for the primary purpose of destruction by fire.  The belly tank’s large fire per unit and intense heat generated is quite dangerous to personnel and its demoralizing effect disrupts temporarily the enemy’s operations.

VI. RECOMMENDATIONS:

1.  Use of bombs heavier than 500 pounds is unnecessary.

2.  Against heavy positions, 250 or 500-pound demolition bombs should be used.

3.  250-pound bombs should be used against medium and light positions only on very close support.  Some near miss damage which temporarily puts positions out of action would be obtained with 250’s that would not be secured with 100’s.  The recoverability of positions is so great that unless an assault is made immediately, all advantages of near miss damage will be lost.  100-pound bombs should be employed against all medium and light positions except in front line positions.

4.  In medium bombardment from altitude under 6,000 feet, 500-pound bombs are necessary against medium and heavy positions.  100-pound bombs should not be used if bombing is from below 10,000 feet.

5.  When consistent with aircraft safety .01 second fuze delay should be used, except on heavy positions where .025 second is recommended.

6.  Longer delays on long delay bombs now be used.

7.  Use of the Mark 13 mine as a bomb be restricted to jungle-clearing operations.

8. Maximum destructive effect on buildings resulted from fire, whether of demolition bomb or incendiary origin.  Most houses in which fires were started were thoroughly burnt out.  The clusters of 6 lb. M69 oil-filled bombs are ideally suited for these targets from bombers.  The M47A2 NP 100-pound bomb is the only standard incendiary available for use with fighters.

9.  It is recommended that the belly tank type of incendiary be used only as an anti-personnel munition for air to ground support, with materiel destruction as its secondary action, and not primarily as a materiel incendiary except as a last resort and then only against highly combustible materials.  From information gained it is believed that even better results can be obtained by the use of the Fuze, M-3, with M15 WP grenade as an igniter adapted to belly tanks filled with Napalm-gasoline mixture, which has a longer burning time and better adhesive properties, and their future use against personnel targets is recommended.

GILBERT B. SHURE

Lt. Col., Ord. Dept.,

Coordination CWS………….. Ordnance Officer.

 10th Air Force