He was the commanding officer of the U.S.S. S 48, based at New London until 2 December 1941, when he was detached to fit out the U. S. S. Growler at the Electric Boat Company. He assumed command of the Growler when she was  commissioned on 20 March 1942 ³. 

Prior to her first war patrol; Growler was assigned picket duty as part of the Hawaii defense force.

The U. S. S. Growler's  [SS215] first war patrol of World War II began on 29 June 1942. She was assigned to patrol around Dutch Harbor, Alaska. Five days into her patrol she saw her first action. Three enemy destroyers were sighted in the harbor. Growler closed on them for a submerged torpedo attack and then surfaced. Her torpedoes hit the first two targets. The first torpedo hit the destroyer, Arare, amidships hitting her boilers causing her to explode. The second torpedo hit the bow of the destroyer Kasumi. The third destroyer had fired two torpedoes at the Growler before the enemy ship, Shiranuhi, was hit in the bow. The two Japanese torpedoes ran down each side of the Growler. The Growler dived deep. The torpedoes missed and no depth charges were sent her way. The destroyer, Arare, was sunk and the other two were severely damaged. They were floatable and brought to Japan for repairs. Lt. Cdr. Gilmore's Navy Cross Citation states that two destroyers were sunk and the third was badly damaged. I could not find confirmation of which statement is true. Growler completed her patrol without firing any more torpedoes and on 17 July 1942 berthed at Pearl Harbor. By skillful handling of his boat he was able to bring his boat home with only minor damages, Lt. Cdr. Gilmore's determination and fine seamanship throughout these operations were in keeping with the highest traditions of the United States Naval Service. Lt. Cdr. Gilmore was awarded the Navy Cross for extraordinary heroism and outstanding devotion to duty as Commanding Officer of a U. S. Submarine on offense war patrol. Growler received her first Navy Unit Citation. ⁴    

³ Navy Department Press Release 7 April 1943, Commander Howard W. Gilmore U.S.N. [Deceased ]

⁴ Subnet- http://www.subnet.com./fleet/ss215.htm --Growler

"For extraordinary heroism and outstanding devotion to duty as Commanding Officer of a U. S. submarine on offensive war patrol. During nine days in enemy controlled waters, Lieutenant Commander Gilmore, by expert maneuvering, boldly brought his submarine into an enemy-controlled harbor where men of war had been sighted and, in rapid succession, attacked three anchored 1700-ton Japanese destroyers with the result that the submarine sank two of these and seriously damaged the third. By skillful handling of his ship he was able to bring his ship home with only minor damages. Lieutenant Commander Gilmore's courage, determination and fine seamanship throughout these operations were in keeping with the highest traditions of the United States Naval Service." ⁵ 

THE U. S. S. Growler's second patrol started on 21 August 1942. Her assigned area was around Taiwan. On 23 August she ran a submerged night attack on a freighter. Her two torpedoes ran under the target and failed to explode. The freighter's quick action running into shallow water prevented the Growler from a surfaced gun attack. On 25 August she worked her way into a large fishing fleet. She sighted and fired three torpedoes at a large passenger freighter. All three torpedoes missed and the Growler suffered a 53 ash can attack, which lasted for three hours. Growler did not sustain any damage and upon surfacing she immediately spotted a convoy. After maneuvering for about two hours she failed to catch up with the main body of the convoy. Her success was that she fired on and sank an ex-gunboat, Senyo Maru. The Growler had no activity in this area for three days, so she shifted her patrol to the east side of the island. On 31 August she sighted the Eifuku Maru, a 5,866 ton cargo ship. This ship was sunk after only 40 minutes from first sighting. On 4 September the Kashino, a 4,000-ton supply ship, was sunk by the Growler's gunfire. On 7 September she fired two torpedoes and sank the 2,204-ton cargo ship, Taika Maru. This ship broke in half and sank in two minutes. On 15 September Growler cleared her patrol area and arrived back at Pearl Harbor on 30 September.   

⁵ Navy Department Press Release 7 April 1943, Commander Howard W.
Gilmore

"For extraordinary heroism as Commanding Officer of a U. S. Submarine during aggressive and successful submarine war patrol. Taking advantage of every favorable attack with alert skill and courageous efficiency, Commander Gilmore succeeded in sinking a total of 25,946 tons of enemy merchant shipping. By expert maneuvering he brought his boat safely through without material damage and his crew home without injury or loss of life. His gallant leadership and resourceful command were in keeping with the highest traditions of the United States Naval Service." Captain Gilmore received a Gold Star in lieu of a second Navy Cross." ⁶ The Growler received her second Navy Unit Citation. 

During a refit for her third war patrol; Growler had new surface radar installed as well as a new 20mm gun. Growler sailed on her new patrol area in the Solomon Islands across the key Truk-Rabaul shipping lines. Her patrol area was during the bitter fighting over Guadalcanal. This area was saturated with enemy planes and only eight enemy ships were sighted but not one attack could be launched. On 3 December 1942, Growler departed from this area and headed for her new homeport of Brisbane, Australia. She arrived on 10 December for refit and recreation.

The Growler's fourth patrol started on New Year's Day 1943. That day saw Growler sail from Brisbane for what was to prove one of the most gallant actions in Naval History. Entering her patrol area on 11 January 1943, again athwart the Truk-Rabaul shipping lanes. Prior to Guadalcanal being secured the " Tokyo Express " operated out of the important island of New Britain. The U. S. Submarines were concentrated on the Bismark Archipelago. This was where the Japanese activity was bubbling over. Rabaul was the most important anchorage for Japan's supply line. Rabaul was where Yamamoto's strongest naval units were accommodated. Rabaul had many guns and saturated with naval installations. This was the beginning of the Tokyo Express. Growler waited only five days before sighting an enemy convoy. Maneuvering inside the escorts, Growler fired two torpedoes and saw them hit. Then, as her war diary reports, she was in the unfortunate position of being about 400 yards from the destroyer and had to dive without being able to attack and successfully evaded a severe depth charge attack. She was credited with the sinking of the Chifuku Maru, a passenger cargo ship of 6,000 ton. On 19 January, Growler sighted and sunk another passenger cargo ship. The Miyadono Maru, also about 6,000 tons, Growler then changed her patrol area and headed for enemy traffic between Turk and Palau. In this area, she damaged a freighter but suffered a heavy depth charge attack.  On 31 January, she attacked a heavily armed gunboat, only to have her life endangered by a defective torpedo.

⁶ Navy Department Press Release 7 April 1943, Commander Howard W. Gilmore

On 4 February, while patrolling south of Steffen Strait, Growler  contacted  a convoy enroute to Rabaul. Capt. Gilmore sighted two merchant ships escorted by two patrol crafts. He calculated the speed and course and started the Growler on an end around so that he would end up ahead of the convoy. This was accomplished but poor visibility made a surface attack his only choice. The Growler was closing on its prey when suddenly the lead ship fired at her. Capt. Gilmore ordered a quick dive and went deep and rigged for depth charge attack. This attack lasted for some time and ruptured a manhole gasket in the forward main ballast tank. This caused a flooding of the forward torpedo room. The damage control party, by placing  a rubber sheet over the manhole and shoring it with jacks, minimized the flow of seawater into the torpedo room. When the attack ended Capt. Gilmore brought his boat to periscope height and saw three of the enemy ships pulling away. This left one patrol craft in his area. Since the drain pump was keeping up with the incoming seawater, Capt. Gilmore held the boat under and headed in the opposite direction. When nightfall came, he surfaced and the necessary repairs to the damages manhole cover were made.⁷   

The patrol continued with two more attacks but no sinking. Shortly after 0100 on 7 February, while charging batteries, Growler stealthily approached a gunboat for a night surface attack. The Hayasaki, a 2,500 ton fighting man-o-war, made especially to combat subs, suddenly turned to ram. Capt. Gilmore sounded the collision alarm and brought the Growler to left full rudder. He intended to avoid both ramming and being rammed. The swinging of the boat to the left put Growler on a collision course and she rammed the enemy amidships at 17 knots, ripping her side plating wide open. Growler heeled over to 50 degrees, bending sideways 18 feet of the bow and disabling the forward torpedo tubes. The Japanese unloaded murderous gunfire, at point blank range, on the conning tower personnel. Instantly killing the JOOD, Ensign W.  Williams and lookout Fireman W. F. Kelley and wounding the Captain. 

The courageous sub seemed lost, Capt. Gilmore cleared the bridge except for himself. The officer of the deck, the quartermaster and two wounded lookouts hurried down the hatch. The executive officer LtCdr. Schade waited at the foot of the ladder for the Skipper to come down. The Captain, desperately wounded, realized that he could not get below in time if his boat were to be saved. "Take her down,'' he ordered.  LtCdr. Arnold F. Schade, who had fallen in the control room, from the ramming, waited for the Captain to leave the bridge. When the Captain did not appear he closed the hatch and the Growler slipped beneath the surface of the water, as Capt. Gilmore, Ensign Williams and Fireman Kelley floated in the sea. ⁸

LtCdr. Schade was now in command of the Growler, with water in the control room and the pump room half flooded leveled off the Growler. He accomplished this although he was badly bruised and in a daze. The crew managed to control the boats flooding. Regaining his full conscience and realizing that the crew had everything under control, he battle surfaced in hopes of finding the enemy. He found and empty sea, gone were Capt. Gilmore, Ensign Williams and Fireman Kelley. The enemy ship, Hayasaki, surviving the engagement had sailed away. 

⁷ The PERISCOPE, January/March 2000, Remembering the USS Growler [ SS 215 ]
⁸ Submarine Sailor- http://www.submarinesailor.com/boats/ss215growler/ 

LtCdr. Schade at this point assessed the damage to the Growler. He found that eighteen feet of the bow was bent at a right angle to port. The outer doors of two forward torpedo doors could not be closed. About 35 feet of the bow, including the bow buoyancy tank, had been crumpled. LCDR. Schade limped back to Brisbane, arriving on 17 February and was taken immediately to dry dock. Commodore Fife praised the performance of the officers and crew in effecting repairs and bringing the boat safely back to base as one of the outstanding submarine feats of the war to date. He said that the Growler will be repaired and will fight again. Once again the Growler received her third Navy Unit Citation. Cdr. Gilmore sacrificed his life for his crew and boat. He wrote another stirring tale of inspirational Naval History. Cdr. Gilmore became the first U.S. Submariner to receive the Congressional Medal of Honor. One of seven submariners to receive this honor. 

[See the Kangaroo Express at http://submarinesailor.com/boats/ss215growler/  ].