Boeing XB - 15 The Boeing XB - 15 ( Boeing 294 ) was a United States bomber aircraft designed in 1934 as a test for the United States Army Air Corps ( USAAC ) to see if it would be possible to build a heavy bomber with a range . For a year beginning in mid - 1935 it was designated the XBLR - 1 . When it first flew in 1937 , it was the most massive and voluminous aircraft ever built in the US . It set a number of load - to - altitude records for land - based aircraft , including carrying a payload to on 30 July 1939 . The aircraft ' s immense size allowed flight engineers to enter the wing through a crawlway and make minor repairs in flight . A flight took 33 hours at its cruising speed ; the crew was made up of several shifts , and bunks allowed them to sleep when off duty . Design and development . The specification that produced the XB - 15 began in mid - 1933 as " Project A " , USAAC discussions regarding the possibility of flying a very large bomber with a range of . In April 1934 the USAAC contracted with Boeing and Martin to design a bomber capable of carrying at over a distance of 5 , 000 miles . Boeing gave the project the internal name of Model 294 , while the USAAC called it the XB - 15 . Martin ' s design , the XB - 16 , was judged inferior by the USAAC before a prototype was built , and was canceled . The Boeing design team , headed by Jack Klystra , initially intended the aircraft to use Allison V - 3420 liquid - cooled W engine s ; since these were not ready , Pratt & Whitney R - 1830 air - cooled radial engines were used instead . Starting in August 1934 , Boeing began designing the Model 299 in answer to a proposal by the USAAC to replace the Martin B - 10 bomber . The Model 299 design team incorporated elements of the Model 294 , especially the use of four engines , and the Boeing 247 . The Model 299 design team worked alongside Klystra ' s team , but difficulties in fabricating such a large aircraft slowed progress on the 294 . The Model 299 flew first , on 28 July 1935 . In mid - 1935 , the USAAC combined Project A with Project D ; a proposal asking for " the maximum feasible range into the future . " The combined program was designated BLR for " Bomber , Long Range " . The XB - 15 was renamed the XBLR - 1 ; it was joined under the BLR program by two other projects : one from Douglas Aircraft , the XBLR - 2 which later became the XB - 19 ; and one from Sikorsky Aircraft called the XBLR - 3 , later canceled . The next year , the XBLR was dropped and the Boeing prototype was once again the XB - 15 . Unusual features that the XB - 15 pioneered included an autopilot , deicing equipment , and two gasoline generators used as auxiliary power unit s ( independent of the main engines ) to power the 110 - volt electrical system . The main engines were serviceable in flight using an access tunnel inside the wing . The aircraft contained a sizable crew compartment with bunkbeds , a galley and a lavatory . Finally , in September 1937 construction was finished , and on 15 October it first flew . Its double - wheel main landing gear remained down from takeoff to landing . On 2 December 1937 , the XB - 15 flew from Seattle to Wright Field in Ohio to be accepted by the USAAC for testing . With the weaker engines installed , the specified speed of 200 mph was not quite reached even when the aircraft was empty ; the best speed attained in level flight was . Loaded with the specified 2 , 000 pounds , the maximum speed was a disappointing . This was considered too slow for a combat aircraft , and the project was abandoned . However , Boeing engineers projected that the prototype would be capable of carrying the heaviest air cargo to date : a load of . The design challenges stemming from the great size of the XB - 15 were difficult to master , but the lessons learned by Boeing were later applied to the Model 314 flying boat . In 1938 , the USAAC proposed to update the XB - 15 to make the slightly larger Y1B - 20 , using more powerful engines , but the Secretary of War , Harry Hines Woodring , canceled the project before construction began , in favor of the expensive Douglas XB - 19 . Boeing went ahead with an internal redesign of the XB - 15 called Model 316 , a very heavy bomber with a high wing , a pressurized cabin and tricycle gear . The Model 316 was not built . The progression of design work starting with the XB - 15 finally bore fruit with the Model 345 presented to the USAAC in May 1940 ; the very heavy bomber which resulted in the Boeing B - 29 Superfortress . Operational history . The single prototype was assigned to the 2nd Bombardment Group at Langley Field , Virginia . Following the 24 January 1939 Chillán earthquake in Chile , the prototype flew a relief mission , carrying medical supplies . Commanded by Major Caleb V . Haynes , the aircraft carried of American Red Cross emergency supplies to Santiago , making only two stops along the way , at France Field in the Panama Canal Zone , and at Lima , Peru . Haynes was awarded the Distinguished Flying Cross and the Order of the Merit of Chile , and the whole crew earned the MacKay Trophy . Haynes piloted the XB - 15 again on 10 June 1939 to return home the body of Mexican flier Francisco Sarabia who had died in a crash in the Potomac River . After flying back from Mexico City , Haynes and his copilot William D . Old undertook flight tests at Fairfield , Ohio , lifting very heavy loads . They used the XB - 15 to lift to a height of , and to , setting two world records . Haynes was awarded certificates issued by the National Aeronautics Association ( NAA ) for an international record for " the greatest payload carried to an altitude of 2 , 000 metres " . The XB - 15 was not very fast for a bomber but it was the fastest aircraft capable of carrying so much weight , and traveling such long distances . In July 1939 , Haynes received certificates from the NAA for the establishment of an international speed record with a payload . The latter performance also established a national closed circuit distance record of . Flying from Langley , the XB - 15 arrived at Albrook Field in Panama on 10 April 1940 and immediately began classified bombing tests of canal lock protections , commanded by Haynes and including Captain Curtis LeMay as navigator and Lieutenant John B . Montgomery as bombardier . Of 150 bombs dropped , only 3 hit the target : a specially made bunker simulating a reinforced machine room . The few hits nevertheless led to improvements in bunker design . In early May , Haynes and LeMay made a survey flight from Panama over the Galapagos islands , the inspection including Baltra Island . Haynes piloted the XB - 15 back to the US , leaving Panama on 11 May 1940 . In late 1940 , the XB - 15 ' s defensive guns were removed at Duncan Field in Florida . Seats were attached so that Lend Lease aircraft ferry crews could be returned after delivery . Cargo aircraft . On 6 May 1943 , the Army Air Forces converted the only prototype into a transport , the aircraft being redesignated XC - 105 . A cargo hoist was mounted , and cargo doors fitted . Its maximum gross weight was increased to . By this time , the aircraft was nicknamed " Grandpappy " by 20th Troop Carrier Squadron airmen . It displayed nose art depicting an elephant carrying on its back a large crate labeled " supplies " . During World War II , the XC - 105 carried freight and personnel to and from Florida , and throughout the Caribbean , based out of Albrook Field beginning in June 1943 . Hundreds of young women were flown in " Grandpappy " from Miami to the Canal Zone to engage in US government work ; these trips were dubbed the " Georgia Peach Run " . " Grandpappy " traveled to the Galapagos , landing on Baltra Island at the same airfield built following the XB - 15 aerial survey of May 1940 . " Grandpappy " ′ s flight crew , reduced to six men , described the aircraft as difficult to fly and service . Two fires and a complete failure of the electrical system occurred in the air . The aircraft was retired on 18 December 1944 , assigned to Panama Air Depot . In June 1945 , it was ordered to be scrapped at Albrook Field in Panama , its engines and internal parts removed along with its vertical stabilizer and rudder . The remaining airframe was deposited at Diablo dump , a swampy landfill southwest of the runway , where it slowly sank from sight . Squatters built shacks on stilts in the swamp , covering the remains . The former dump is now an industrial area , with " Grandpappy " underneath . During its 18 months of transport service , the XC - 105 carried more than 5 , 200 passengers , of cargo and of mail . It flew 70 cargo trips and 60 missions including anti - submarine patrol . Unusually , the aircraft was consistently referred to as " he " by its crew .