• Change in Glycosylated Haemoglobin (HbA1c) [ Time Frame: From baseline week 0 (V2) to week 16 (V18) ]
  Estimated mean change from baseline (week 0) in HbA1c at week 16 is presented. The endpoint was evaluated based on the data from the on-treatment without rescue medication observation period, which was the time period when a participant was on treatment with trial product, excluding any period after initiation of a non-randomised insulin treatment (rescue medication).
• Change in Fasting Plasma Glucose (FPG) [ Time Frame: From baseline week 0 (V2) to week 16 (V18) ]
  Estimated mean change from baseline (week 0) in FPG at week 16 is presented. The endpoint was evaluated based on the data from the on-treatment without rescue medication observation period, which was the time period when a participant was on treatment with trial product, excluding any period after initiation of a non-randomised insulin treatment (rescue medication).
• Change in Body Weight [ Time Frame: From baseline week 0 (V2) to week 16 (V18) ]
  Estimated mean change from baseline (week 0) in body weight at week 16 is presented. The endpoint was evaluated based on the data from the on-treatment without rescue medication observation period, which was the time period when a participant was on treatment with trial product, excluding any period after initiation of a non-randomised insulin treatment (rescue medication).
• Weekly Insulin Dose [ Time Frame: During the last 2 weeks of treatment (week 15 and 16) ]
  Estimated mean average weekly insulin dose during the last 2 weeks of treatment is presented. The endpoint was evaluated based on the data from the on-treatment without rescue medication observation period, which was the time period when a participant was on treatment with trial product, excluding any period after initiation of a non-randomised insulin treatment (rescue medication).
• Number of Treatment-emergent Adverse Events (TEAEs) [ Time Frame: From baseline week 0 (V2) to week 21 (V20) ]
  An adverse event(AE) is any untoward medical occurrence in a clinical trial subject administered or using a medicinal product, whether or not considered related to the medicinal product or usage.. A TEAE was defined as an event that had onset date (or increase in severity) during the on-treatment observation period. The on-treatment observation period was the time period from first dose of trial product until the follow-up visit or the last date on trial product + 5 weeks for once daily insulin and +6 weeks for once weekly insulin. Safety analysis set (SAS) included all subjects exposed to at least one dose of trial product.
• Number of Severe Hypoglycaemic Episodes (Level 3) [ Time Frame: From baseline week 0 (V2) to week 16 (V18) ]
  Severe hypoglycaemic episodes (level 3) were defined as episodes that were associated with severe cognitive impairment requiring external assistance for recovery. Number of severe hypoglycaemic episodes that occurred during weeks 0-16 are presented.
• Number of Clinically Significant Hypoglycaemic Episodes (Level 2) (Below 3.0 mmol/L (54 mg/dL), Confirmed by BG Meter) or Severe Hypoglycaemic Episodes (Level 3) [ Time Frame: From baseline week 0 (V2) to week 16 (V18) ]
  Clinically significant hypoglycaemic episodes (level 2) were defined as episodes that were sufficiently low to indicate serious, clinically important hypoglycaemia with plasma glucose value of <3.0 mmol/L (54 mg/dL). Severe hypoglycaemic episodes (level 3) were defined as episodes that were associated with severe cognitive impairment requiring external assistance for recovery. Number of clinically significant hypoglycaemic episodes (level 2), confirmed by blood glucose (BG)meter or severe hypoglycaemic episodes (level 3) that occured during weeks 0-16 are presented.
• Number of Hypoglycaemic Alert Episodes(Level 1) (Greater Than or Equal to 3.0 and Below 3.9 mmol/L (Greater Than or Equal to 54 and Below 70 mg/dL), Confirmed by BG Meter) [ Time Frame: From baseline week 0 (V2) to week 16 (V18) ]
  Hypoglycaemia alert value (level 1) was defined as episodes that were sufficiently low for treatment with fast-acting carbohydrate and dose adjustment of glucose-lowering therapy. Number of hypoglycaemic alert episodes (level 1) (equal to or above 3.0 and below 3.9 mmol/L (equal to or above 54 and below 70 mg/dL), confirmed by BG meter) that occured during weeks 0-16 are presented.

• Change in Glycosylated Haemoglobin (HbA1c) [ Time Frame: From baseline week 0 (V2) to week 16 (V18) ]
  Measured in %-point
• Change in Fasting Plasma Glucose (FPG) [ Time Frame: From baseline week 0 (V2) to week 16 (V18) ]
  Measured in mmol/l
• Change in Body Weight [ Time Frame: From baseline week 0 (V2) to week 16 (V18) ]
  Measured in kg
• Weekly Insulin Dose [ Time Frame: During the last 2 weeks of treatment (week 15 and 16) ]
  Measured in U
• Number of Treatment-emergent Adverse Events (TEAEs) [ Time Frame: From baseline week 0 (V2) to week 21 (V20) ]
  Count of events
• Number of Severe Hypoglycaemic Episodes (Level 3) [ Time Frame: From baseline week 0 (V2) to week 16 (V18) ]
  Count of events
• Number of clinically significant hypoglycaemic episodes (level 2) (less than 3.0 mmol/L (54 mg/dL), confirmed by blood glucose (BG) meter) or severe hypoglycaemic episodes (level 3) [ Time Frame: From baseline week 0 (V2) to week 16 (V18) ]
  Count of events
• Number of hypoglycaemic alert episodes (level 1) (greater than or equal to 3.0 mmol/L and less than 3.9 mmol/L (greater than or equal to 54 mg/dL and less than 70 mg/dL, confirmed by BG meter) [ Time Frame: From baseline week 0 (V2) to week 16 (V18) ]
  Count of events

• Drug: Insulin icodec
  Participants will receive subcutaneous (s.c.) injections of Insulin 287 OW for 16 weeks.
  Other Name: Insulin 287
• Drug: Insulin glargine U100
  Participants will receive s.c. injections of insulin glargine OD for 16 weeks

• Experimental: Insulin 287 (with 100% loading dose)
  Participants will receive insulin 287 injections once weekly (OW). A unit to unit switch approach with an additional 100% loading dose of insulin 287 will be used.
  Intervention: Drug: Insulin icodec
• Experimental: Insulin 287 (without loading dose)
  Participants will receive insulin 287 injections OW. A unit to unit switch approach without loading dose of insulin 287 will be used.
  Intervention: Drug: Insulin icodec
• Active Comparator: Insulin glargine U100
  Participants will receive insulin glargine U100 once daily (OD).
  Intervention: Drug: Insulin glargine U100

Inclusion criteria:

• Male or female, aged 18-75 years (both inclusive) at the time of signing informed consent.
• Diagnosed with type 2 diabetes mellitus greater than or equal to 180 days prior to the day of screening.
• Glycosylated haemoglobin (HbA1c) of 7.0-10.0% (53.0-85.8 mmol/mol) (both inclusive) as assessed by central laboratory.
• Treated with once daily or twice daily basal insulin analogue (insulin degludec, insulin detemir, insulin glargine U100 or U300, total daily dose of 10-50 U, both inclusive) greater than or equal to 90 days prior to the day of screening.

• Stable daily dose(s) for 90 days prior to the day of screening of any of the following antidiabetic drug(s) or combination regime(s):

  1. Any metformin formulations greater than or equal to 1500 mg or maximum tolerated or effective dose (as documented in subject's medical records).
  2. Free or fixed combination therapy: Metformin as outlined above with or without dipeptidyl peptidase 4 inhibitors (DPP4i) with or without sodium-glucose cotransporter 2 inhibitors (SGLT2i) is allowed: 1) DPP4i (greater than or equal to half of the maximum approved dose according to local label or maximum tolerated or effective dose); 2) SGLT2i (greater than or equal to half of the maximum approved dose according to local label or maximum tolerated or effective dose.
• Body mass index (BMI) less than or equal to 40.0 kg/m^2.

Exclusion criteria:

• Known or suspected hypersensitivity to trial product(s) or related products.
• Female who is pregnant, breast-feeding or intends to become pregnant or is of child-bearing potential and not using an adequate contraceptive method.
• Participation in any clinical trial of an approved or non-approved investigational medicinal product within 90 days before screening.
• Any disorder, except for conditions associated with type 2 diabetes mellitus, which in the investigator's opinion might jeopardise subject's safety or compliance with the protocol.
• Any episodes of diabetic ketoacidosis within the past 90 days prior to the day of screening and between screening and randomisation.
• Known hypoglycaemic unawareness as indicated by the Investigator according to Clarke's questionnaire question 8.
• Recurrent severe hypoglycaemic episodes within the last year as judged by the Investigator.
• Myocardial infarction, stroke, hospitalisation for unstable angina pectoris or transient ischaemic attack within 180 days prior to the day of screening and between screening and randomisation.