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===Assembly===
[[File:Nagasakibomb.jpg|thumb|Mushroom cloud after Fat Man exploded over [[Nagasaki]] on 9 August 1945]]
The first plutonium core was transported with its polonium-beryllium modulated neutron initiator in the custody of [[Project Alberta]] courier [[Raemer Schreiber]] in a magnesium field carrying case designed for the purpose by Philip Morrison. Magnesium was chosen because it does not act as a tamper.{{sfn|Coster-Mullen|2012|p=45}} It left [[Kirtland Air Force Base|Kirtland Army Air Field]] on a [[C-54]] transport aircraft of the [[509th Composite Group]]'s 320th Troop Carrier Squadron on 26 July and arrived at [[North Field (Tinian)|North Field]] on [[Tinian, Northern Mariana Islands|Tinian]] on 28 July. Three Fat Man high-explosive pre-assemblies (designated F31, F32, and F33) were picked up at Kirtland on 28 July by three B-29s: ''[[Luke the Spook]]'' and ''[[Laggin' Dragon]]'' from the 509th Composite Group's [[393d Bombardment Squadron]], and another from the [[216th Army Air Forces Base Unit]]. The cores were transported to North Field, arriving on 2 August, when F31 was partly disassembled in order to check all its components. F33 was expended near Tinian during a final rehearsal on 8 August. F32 presumably would have been used for a third attack or its rehearsal.{{sfn|Campbell|2005|pp=38–40}}

On 7 August, the day after the bombing of Hiroshima, [[Rear Admiral (United States)|Rear Admiral]] [[William R. Purnell]], [[Commodore (United States)|Commodore]] [[William S. Parsons]], Tibbets, [[General (United States)|General]] [[Carl Spaatz]] and [[Major General (United States)|Major General]] [[Curtis LeMay]] met on Guam to discuss what should be done next.{{sfn|Russ|1990|pp=64–65}} Since there was no indication of Japan surrendering,{{sfn|Frank|1999|pp=283–284}} they decided to proceed with their orders and drop another bomb. Parsons said that Project Alberta would have it ready by 11 August, but Tibbets pointed to weather reports indicating poor flying conditions on that day due to a storm and asked if the bomb could be made ready by 9 August. Parsons agreed to try to do so.{{sfn|Russ|1990|pp=64–65}}{{sfn|Groves|1962|p=342}}

Fat Man F31 was assembled on Tinian by Project Alberta personnel,{{sfn|Campbell|2005|pp=38–40}} and the physics package was fully assembled and wired. It was placed inside its ellipsoidal aerodynamic bombshell, which was painted mustard yellow, and wheeled out, where it was signed by nearly 60 people, including Purnell, Brigadier General [[Thomas F. Farrell]], and Parsons.{{sfn|Coster-Mullen|2012|p=67}}<ref name=":0">{{Cite magazine |last=Wellerstein |first=Alex |date=2015-08-07 |title=What About Nagasaki? |url=https://www.newyorker.com/tech/annals-of-technology/nagasaki-the-last-bomb |access-date=2024-07-28 |magazine=The New Yorker |language=en-US |issn=0028-792X}}</ref> The acronym "JANCFU" was stenciled on the bomb's nose, standing for "Joint Army-Navy-Civilian Fuckup", a play on the acronym "[[SNAFU]]".<ref name=":0" /><ref>{{Cite web |title=World War II: the battle for Kiska (pt 2). - Free Online Library |url=https://www.thefreelibrary.com/World+War+II:+the+battle+for+Kiska+(pt+2)-a030243144 |access-date=28 July 2024 |website=www.thefreelibrary.com |quote=It was the Battle of Kiska that would lead Time magazine to create the acronym, JANFU (joint army-navy foul-up) to complement the earlier SNAFU (situation normal, all fouled-up).}}</ref><ref>{{Cite journal |first1=Ellen |last1=Bradbury |first2=Sandra |last2=Blakeslee |date=5 August 2022 |title=The harrowing story of the Nagasaki bombing mission |url=https://thebulletin.org/2022/08/harrowing-story-of-the-nagasaki-bombing-mission/ |access-date=28 July 2024 |journal=Bulletin of the Atomic Scientists |quote=It even had a code: JANCFU for "joint army navy combined foul up" which was a cousin of "SNAFU", military vernacular for "situation normal, all f***ed up."}}</ref> It was then wheeled to the [[bomb bay]] of the B-29 Superfortress named ''[[Bockscar]]''<!-- yes, it is "Bockscar" not "Bock's Cr". This is ''not'' a typo! --> after the plane's command pilot Captain [[Frederick C. Bock]],<ref>{{cite web|url=https://awesometalks.wordpress.com/2008/08/07/bockscar-the-forgotten-plane-that-dropped-the-atomic-bomb/ |title=Bockscar … The Forgotten Plane That Dropped The Atomic Bomb « A Little Touch of History |date=7 August 2008 |publisher=Awesometalks.wordpress.com |access-date=31 August 2012}}</ref> who flew ''[[The Great Artiste]]'' with his crew on the mission. ''Bockscar'' was flown by Major [[Charles W. Sweeney]] and his crew, with Commander [[Frederick L. Ashworth]] from Project Alberta as the weaponeer in charge of the bomb.{{sfn|Campbell|2005|p=32}}